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Human Origins
The Evolution of Mankind |
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There are many causes for variation in organisms. Variations that are inherited form the basis for the origin of new species; individuals that share common characteristics and can mate to produce fertile offspring. In describing human origins, it is appropriate to begin with our ancestral biological group, the mammals.
Mammals
The earliest mammals were small, warm-blooded, nocturnal, egg-laying quadrupeds that shared the land with the dinosaurs and mammal-like reptiles beginning about 200 million years ago, at the start of the Jurassic period. They lived in a world without social insects, in which ferns and conifers thrived, and where the two large supercontinents, Laurasia and Gondwana, were just starting to break up. These earliest mammals were ancestral to the three groups of mammals that exist today: monotremes (platypus, echidna), marsupials (including kangaroos, koalas, and possums), and placental mammals.
Only monotremes are egg-laying; the offspring of marsupials and placental mammals are born live. Marsupial females carry their maturing young in abdominal pouches. The offspring of placental mammals are mature enough to live separate from their mothers from birth, but require support throughout infancy.
Mammals were distinguished from dinosaurs in several ways. Dinosaurs were primarily diurnal, while early mammals were mostly nocturnal. Mammals were hairy, whereas dinosaur skin had scales or was covered by feathers. Mammal females have glands that produce milk, used to nourish their young. In mammals, two reptilian mandibular bones, the lower jaw articular and the upper jaw quadrate, evolved into two small bones of the middle ear, the malleus and the incus. In conjunction with a third small bone, the stapes, the malleus and the incus connect the eardrum to the inner ear and function to transmit and amplify sound.
Middle ear modifications in the skull and the development of ears gave mammals a superior hearing capability useful in their nocturnal life. But adaptation to a dark environment led to loss of some daytime vision capabilities, particularly to degradation in the quality of color vision. The retinas of mammals have fewer types, and a smaller number, of color-sensitive cone cells than the retinas of reptiles and the dinosaur descendants, the birds.
Mammalian brains evolved to incorporate a new structure, the neocortex, absent from dinosaurs and all reptiles. The neocortex supports sensory perception and reasoning.
Eomaia scansoria was an early eutherian mammal, transitional between marsupial and placental mammals. A small hairy creature (about 5 inches long, one ounce in weight), it was adapted to climbing bushes and trees.
Scansoria was likely insectivorous. It lived 125 million years ago in what is today northeastern Asia and was ancestral to all placental mammals (including primates, dogs, horses, and mice).
Flowering plants evolved during the Cretaceous and became widespread around 100 million years ago. Insects involved in pollination, such as bees and wasps, co-evolved with the flowering plants. Flowering shrubs and trees increasingly appeared in mammal habitats.
Early Primates
About seventy-five million years ago, late in the Cretaceous period, the large supercontinents that had earlier existed were completing their breakup into the current continents, although the new continents had not yet drifted to their present locations and were shaped somewhat differently than today. The climate was warmer and more humid than today. Average temperatures were about 10 deg F higher than at present, and atmospheric oxygen content was lower, about 18% rather than the present 21%.
Alongside the predominant dinosaurs, there lived small placental mammals, the earliest primates. These earliest primates looked sort of like squirrels, and lived mostly by eating insects and staying away from carnivorous dinosaurs. During the Paleocene epoch, 75 to 65 million years ago, arboreal early primates appeared that had grasping hands and feet.
The Cretaceous-Tertiary Extinction
Sixty-five million years ago, an asteroid about six miles wide hit the Earth in the area of present-day Yucatan. The impact melted the local crust, created a crater about 110 miles wide, and blasted millions of tons of vaporized and molten debris into the atmosphere, darkening the skies for months. This disastrous event caused massive destruction worldwide and led to the extinction of the majority of lifeforms on the planet: all large vertebrates, many land plants and tropical invertebrates, and most plankton. Some sea-dwelling creatures (fishes, mollusks, corals) survived, as did birds, mammals, and some reptiles and amphibians.
Geologic Time
Geologic time is divided into periods. The Cretaceous period covers the time from the end of the Jurassic, about 146 million years ago, to 65 million years ago. It is followed by the Terciary period, which covers the time from about 65 million years ago to 2.6 million years ago. The Quaternary period covers the time from about 2.6 million years ago to the present.
Geologic periods are divided into epochs. The Terciary period is divided into five epochs: Paleocene, Eocene, Oligocene, Miocene, and Pliocene. The Quaternary period has two epochs, the Pleistocene, and the current epoch, the Holocene.
Primate Expansion
Some of the smaller creatures, nook dwellers with modes of existence that made them familiar with hiding and seeking cover, found it easier to survive the Cretaceous-Tertiary disaster. The early primates were among the species that lived on past the end of the Cretaceous period. After the climate recovered from the effects of the asteroid impact, the mass extinctions opened up many ecological niches. The absence of large predators and herbivores, including all the dinosaurs, facilitated the expansion of the early primates.
Major evolutionary changes occurred in some of the primates during the Eocene epoch, 55 to 34 million years ago. Species appeared that resembled some current species, like lemurs and tarsiers, and possessed larger brains. They scurried along tree branches using their grasping hands and feet, their tails helping them to balance themselves when leaping small gaps between tree limbs. The position of the eyes had changed from the sides of the head toward the front, allowing stereoscopic vision for a greater portion of the field of view. Stereoscopic vision allowed the accurate estimation of distance, essential for leaping from branch to branch, and provided an evolutionary advantage.
In some species, the location of the foramen magnum, which indicates the juncture of the spinal column and the head, shifted from the back of the skull toward the bottom. The shift in the position of the foramen magnum suggests that these early primates were often holding their bodies erect, as modern lemurs do while sitting.
The expansion of the primates led to entry into daytime environments where color perception grew in importance. In brightly illuminated areas, color discrimination would result in enhanced capabilities to identify edible fruits and leaves. Adaptation to daylight caused most primates to evolve trichromatic vision.
Grasses, flowering plants that pollinate by wind, began to expand during the Eocene, and made up increasingly larger portions of the vegetation cover of the Earth.
By the end of the Eocene, photosynthesis from worldwide increases in vegetation had resulted in a rise in atmospheric oxygen, to about 23%. Average worldwide temperatures were about 6 deg F higher than at present. At the time, Africa was a separate continent, without land links to Eurasia or South America.
About 37 million years ago, the primates were split into two branches that evolved separately, the prosimians and the anthropoids. The prosimians evolved into the present-day lemurs, eye-eyes, lorises, bush babies, and tarsiers.
Monkeys
Anthropoid primates resembling some current monkeys first appeared about 37 million years ago, near the end of the Eocene epoch. Early anthropoids were arboreal and vegetarian. They lived in forests, ate mostly seeds and fruits, and had tails, shorter snouts and forward-looking eyes. During the Oligocene epoch, 34 to 23 million years ago, newly-evolved monkey species replaced many of the older primates, the prosimians. Some early monkeys, like Aegyptopithecus, were larger than their prosimian ancestors and weighed about 16 pounds.
Anthropoids developed independently in the separated African and South American continents. Two types emerged, the New World monkeys, the platyrrhini, and the Old World monkeys, the catarrhini.
The platyrrhini evolved into the present-day monkeys of South America: marmosets, tamarins, and capuchins. New World monkeys are more primitive than Old World monkeys. The thumbs on the platyrrhini’s long, narrow hands are not opposable, and their brains are less complex. The catarrhini evolved into the apes, and the present-day African baboons, macaques, and colobus monkeys.
Monkeys use various modes of locomotion. On horizontal surfaces they typically use quadrupedal locomotion. They use their arms to swing from one tree branch to another. When looking about, they may stand or walk on two feet, using the tail or arms to hold on to a support.
Fossils
Fossils are the preserved remains of organisms from the distant past. Soft tissues and microscopic organisms are seldom preserved in the fossil record. More often, fossils are formed from vertebrate bones and teeth, and the exoskeletons of arthropods. Most very ancient bones are not found as bones, per se, but as fossils. The fossils have the shape of bones, but the bone material, mostly calcium, is replaced by stone. This happens when bones are buried in sediment that, over a period of thousands of years, gradually replaces the calcium, turning the bones to stone.
Early Apes
The Miocene epoch, starting 23 million years ago, brought further displacement of tectonic plates and creation of new mountain ranges. The global climate became colder and the polar ice caps grew, lowering the sea level. Lower temperatures led to the spread of grasslands and to the fragmentation of forest habitats. The climactic and vegetation changes coincided with the evolution of new anthropoids in Africa.
About 22 million years ago a tree-dwelling anthropoid species appeared, with legs and arms of equal length, a short face, no tail, and gripping hands and feet. This species, Proconsul, was an early ape. It moved on the trees using all four limbs, gripping branches with hands and feet. The fossil record indicates that Proconsul existed until the middle Miocene, about 15 million years ago. Specimens vary in size. These animals weighed from 25 to 80 pounds.
Fossil remains of an extinct ape, Dryopithecus, have been found in Africa, where it likely originated, and in Europe and Asia. Dryopithecus had a body length of about two feet, and used its arms to swing from branch to branch. If it found it necessary to move on the ground, it would move on all fours, but using the palms of his hands, like a monkey. Like all apes, and unlike monkeys, Dryopithecus had no tail.
Fossils of the ape Sivapithecus have been found in Africa, Europe, India and Asia. Sivapithecus was about 4 feet, 10 inches tall, and existed from about 12.5 to 8.5 million years ago. It ate fruits, was primarily arboreal, and had eyes narrowly set apart. The ape Ramapithecus, closely resembling Sivapithecus and possibly the same species, existed as recently as 5 million years ago. Ramapithecus was mostly a tree-dweller, although it could move on the ground if necessary. Its weight was about 40 pounds. Ramapithecus may be an ancestral relative of present-day orangutans.
Louis de Bonis, George D. Koufos and others have documented their analysis of fossil remains of a large ape that lived in southeastern Europe 8 to 11 million years ago, in what was then a savannah-like environment. Skull fragments, mandibles and teeth of this creature, Ouranopithecus macedoniensis, have been found at Nikiti, Ravin de la Pluie and Xirochori in northern Greece between 1974 and 2004. Ouranopithecus has more human-like characteristics than Dryopithecus or Sivapithecus. Based on the size of available fossil fragments, Ouranopithecus is estimated to have weighted from 110 to 180 pounds. It may be a common ancestor to both later African hominids like Sahelanthropus tchadensis and Ardipithecus ramidus and to the contemporary African great apes.
Hominid Transitional Forms
Sahelanthropus tchadensis was a type of ape that lived in north-central Africa about seven million years ago. A Sahelanthropus cranium, a partial jaw, and some teeth were found at Toros-Menalia in Chad, in 2001-2002. Sahelanthropus had a brain volume of about 350 cc. Its brow ridges and small canine teeth are similar to those of later apes possibly ancestral to humans. The location of the foramen magnum is not certain due to fossil distortion, but the apparent position toward the back of the Sahelanthropus skull is not consistent with habitual upright walking. Sahelanthropus lived at a time when hominids (the family of humans and their extinct ancestors) may have diverged from the line that led to present-day gorillas and chimpanzees.
Fossil bones of an ape, Orrorin tugenensis, found in Kenya in 2000, are consistent with both bipedalism and tree-climbing capabilities. Orrorin lived about six million years ago, in what was then a forest environment, and subsisted on fruits and vegetables. It used its prehensile hands for locomotion in the trees. Upright locomotion may have been used occasionally to move along horizontal branches or to cover open ground in breaks in the forest canopy.
For ground locomotion, contemporary large apes use their four extremities in either a fist-walking (orangutans) or knuckle-walking (chimpanzees and gorillas) gait, but ancient apes appear to have simply kept their hands open when moving on all fours on the ground.
A worldwide cooling trend accompanied the start of the Pliocene epoch 5.3 million years ago. The land bridge between North and South America formed during this epoch, and the polar ice caps expanded. Savannahs spread where previously there had been mostly forests. Atmospheric oxygen content was about the same as the present level of 21%. Average worldwide temperatures were about 2 deg F higher than at present.
At the time, the East African environment was lush and green. There were lakes and rivers with groups of animals living in the water and along the shores. Patches of open grasslands began to appear in the dense forests, where the apes lived. In the forest, apes could move very efficiently by a form of locomotion called brachiation, grasping tree branches and swinging from branch to branch. They used their grasping hands and feet when moving vertically, climbing up and down trees. Some apes would temporarily assume an upright posture and move along horizontal tree limbs by walking on their feet while steadying themselves using their upper limbs.
Most apes remained in the safety of the forest, but some ventured away from the woods, and risked encounters with predators to cross the open ground and reach outlying trees. The changing climate caused some species to die out, and others to take their place.
Ardipithecus ramidus
In 1992-1993, a research team led by American paleoanthropologist Tim White found fossil remains of an ape later designated Ardipithecus ramidus in the Afar region in Ethiopia. Ramidus lived in forested plains in eastern Africa 5.8 to 4 million years ago. It was about 4 feet tall, and its weight was about 110 pounds. Its dentition was ape-like, but it had canines and other features like those of later hominids. Ramidus had a brain volume of about 350 cc. Its cranial bones show a relatively forward position of the foramen magnum, which is consistent with the skull being located in a way associated with bipedalism. Lower limb bones also suggest Ardipithecus ramidus was capable of walking upright, but the big toe was not forward-pointing and the upper body indicates a tree-climbing capability was retained. Ramidus likely could not walk for long distances.
Ramidus fossil remains are fairly complete. They show arms and legs of similar lengths, like Proconsul, and unlike brachiating apes that have arms longer than their legs, or early humans, with legs longer than their arms. The hands of Ardipithecus ramidus were configured for grasping and suggest that locomotion on the trees was often quadrupedal, moving along branches using hands and feet. The hands lacked opposable thumbs. Ramidus did not have the precision grip that is achieved by holding an object securely between the thumb pad and one or more fingers.
At a time when the thinning out of forests often required moving on the ground to reach food sources, bipedalism provided a more efficient mode of locomotion than the form of quadrupedalism used by apes. Unlike the efficient four-footed locomotion of committed quadrupeds like cats and horses, the quadrupedal walking used by apes is cumbersome and uneconomical. The evolutionary advantages gained by walking on two legs must have been substantial, because bipedalism is a complex adaptation, involving significant changes to the structure and arrangement of foot, leg, spine and hip bones, and to the musculature.
The Australopithecines
The term australopithecus (southern ape) is applied to a range of extinct African ape-like creatures that play a key role in the explanation of the origins of humans.
Australopithecus anamensis
In 1965, a research team from Harvard University found an australopithecine arm bone in the East Lake Rudolf region that was estimated to be about four million years old. Later, additional specimens of the same type, including a complete lower jaw, were found near Allia Bay in the Lake Turkana region. (The name of Lake Rudolf, a large salty lake in Kenya, was changed to Lake Turkana.) In 1995, paleoanthropologist Maeve Leakey and her group named the species represented by these bones Australopithecus anamensis. In 2006, additional anamensis fossils were found north of the Lake Rudolf site, in Ethiopia.
Fossil remains of Australopithecus anamensis, dating from 4.2 to 3.9 million years ago, are consistent with upright locomotion. Australopithecus anamensis apparently lived in a varied environment in east central Africa that included lake shore, bush, woods, and grassy open terrain.
The dentition of Australopithecus anamensis was ape-like, similar to that of Ardipithecus ramidus. Leg bones of anamensis have features that indicate bipedalism and which are preserved in humans.
Australopithecus afarensis
In 1934, anthropologist Ludwig Kohl-Larsen discovered an australopithecine jaw at Laetoli in German East Africa (later Tanzania). In 1973, Donald Johanson found a knee joint of the same type in the Afar region in Ethiopia, and a year later Johanson and Tom Gray found an incomplete skeleton nearby. In 1978, Mary Leakey discovered preserved footprints in the Laetoli region. The footprints showed big toes in line with the rest of the toes, and an arch between the ball of the foot and the heel, like in humans. They had been made 3.7 million years ago by bipedal hominids. The species represented by these fossil remains has been named Australopithecus afarensis.
Australopithecus afarensis lived in eastern Africa between 4 and 2.7 million years ago. Its dentition and cranium were ape-like but had distinct hominid features. Fossil remains and footprint trails associated with Australopithecus afarensis are those of bipedal individuals that retained some characteristics of tree-dwellers. The hands of Australopithecus afarensis were capable of holding objects securely between the thumb and finger tips, but there is no evidence that afarensis used tools. Brain size was about 450 cc.
With a diet of fruit and vegetables, afarensis would need a relatively large stomach and intestine to process a large volume of foods with fairly low caloric content. The abdomen of afarensis was protuberant, lacking a waist. It was heavily built for its height and had long arms and short legs. Afarensis fossils indicate a height of from 3 ft, 6 in to 5 ft, and weight of 65 to 155 pounds. Although well adapted for walking, Australopithecus afarensis was not designed for running. There was significant sexual dimorphism, with males being substantially larger.
Australopithecus africanus
In 1924, Australian anatomist Raymond Dart discovered at Taung, in South Africa, the skull and a few bone fragments of a creature sharing human and ape-like features. Dart named the hominid Australopithecus africanus.
Africanus dentition and eye orbits had some human-like characteristics, and the foramen magnum at the base of the skull indicated an upright posture. Other Australopithecus africanus fossils have since been found at Sterkfontein, Makapansgat and Gladysvale in southern Africa. They indicate a height of from 3 ft, 7 in to 4 ft, 7 in, and weight of 65 to 135 pounds. Africanus first appeared about 3 million years ago. Its brain size was about 500 cc.
The pelvis of Australopithecus africanus was better adapted to bipedalism than that of Australopithecus afarensis. Living on the plains, often away from the shelter of trees, africanus likely used clubs and stones to fend off predators. The dentition of Australopithecus africanus indicates that it consumed fruits and vegetables, but its diet was enhanced by occasionally eating meat from small animals.
In 2011 and 2012, Sonia Harmand and Jason Lewis of Stony Brook University, New York, found very primitive stone tools west of Lake Turkana, in Kenya. The hand hammer-like objects had flat surfaces that may have been used for cracking nuts or bone. No edged tools suitable for cutting were found at that site. Harmand, Lewis and others described results of their analyses in a paper published in Nature on May 21, 2015. They dated the tools to between 3.1 and 3.3 million years ago and estimated that the local environment at that time was partially shrubby and woody. No hominid bone fossils were found in association with the stone tools, but the dating of the tools suggests that either a late Australopithecus afarensis, early Australopithecus africanus, or an as yet undiscovered hominid species fashioned these earliest tools.
Crude scrapers and choppers, among the first stone tools, but more advanced than those found by Harmand, appeared about 2.5 million years ago. They were made by hitting a stone with another to break off a sharp-edged flake. Chemical analysis of bone collagen and tooth enamel, using stable isotopes of the elements carbon and nitrogen, have yielded estimates indicating the diet of Australopithecus africanus included animal protein. Before Australopithecus africanus disappeared about 2 million years ago, it may have occasionally hunted small game and scavenged in the kill sites of non-human predators.
Late Pliocene Climate Changes
About 2.4 million years ago, worldwide cooling trends led to the reglaciation of Antarctica and the growth of Polar ice sheets. Average global temperatures were then about 5 deg F lower than at present. In some areas of Africa, receding forests were replaced by grasslands. In the habitat of the Australopithecines, fruit-bearing trees became more scarce.
Australopithecus robustus
Sometime around 2.2 million years ago, a line of australopithecines developed distinct adaptations to a vegetarian diet consisting primarily of tough plant foods available in the savannah, such as nuts, tubers, and hard fruits. They evolved heavy skulls supporting massive jaws and molars, and incorporating a sagittal crest to anchor their powerful jaw muscles. Fossil remains of this type were first found in 1938 in Kromdraai, South Africa, by a boy called Gert Terblanche, who took them to Robert Broom. More bones of the type were later found, and Broom, a physician and paleontologist, named the species they belonged to Australopithecus robustus.
Other australopithecines, referred to as gracile, had significantly smaller jaws and molars, and lacked a sagittal crest. Like other australopithecines, Australopithecus robustus had its foramen magnum at the center of the base of the skull, indicating it was a biped. Weight was about 100 pounds, and brain size was about 525 cc. An east African variety of robustus has been designated Australopithecus boisei. The extreme adaptation to vegetarianism found in robustus allowed that species to get along in south and east Africa until a million years ago, when it became extinct.
Australopithecus sediba
A team led by Lee R. Berger of the University of Witwatersrand, South Africa, and Paul H. Dirks of James Cook University, Australia, announced in 2010 the identification of a previously unknown type of hominid. In 2008, Lee Berger brought along his 9-year-old son, Mathew, to an exploration of a cave-rich area in Malapa, about 25 miles from Johannesburg. Mathew promptly found a hominid clavicle fossil. Later explorations by Berger’s team found more fossils in the same area. The fossil bones were assembled into partial skeletons of an adult male and female, and a male juvenile about 12 years old.
Studies of the fossil remains and the ground they were found in indicated the hominids had lived in an environment of wooded valleys, grassy plains and rivers. The species, Australopithecus sediba, had relatively long arms and legs. The adult female weighed about 73 pounds and was about 4 feet, 2 inches tall. The cranium was small and the structure of the pelvis was consistent with upright walking. The juvenile’s brain volume was about 435 cc. Sediba’s small teeth and facial structure was more human-like than other gracile Australopithecines.
The Malapa remains were dated at between 1.78 and 1.95 million years ago, which suggests Australopithecus sediba lived after Australopithecus africanus and overlapped in time with Australopithecus robustus and Homo habilis.
Homo habilis
About 2.2 million years ago, at about the same time as the first appearance of Australopithecus robustus, a new creature appeared, displaying more human-like characteristics than any preceding it. Fossil remains of this type of hominid, Homo habilis, were first discovered in 1960 by Jonathan Leakey at Olduvai Gorge in east Africa. In 1986, Donald Johanson and Tim White found additional fossils at Olduvai Gorge, including arm and leg bones.
The ancestors of Homo habilis dealt with late Pliocene climate changes in a daring manner. They risked encounters with saber-toothed cats, hyenas, and other predators by increasingly becoming predators themselves. Habilis supplemented nourishment from gathering fruits, berries and nuts, by also scavenging and hunting.
Homo habilis had a slimmer build and smaller teeth than any of the australopithecines. Significantly, habilis also had a larger brain of 600 to 800 cc, and hands with fully opposable thumbs. Its remains are often accompanied by simple stone tools: choppers, hammer stones, scrapers and flakes. Homo habilis (dexterous man) was typically 5 feet tall and weighed about 110 lbs.
Very early stone tools have been associated with Homo habilis and are termed Oldowan, after Olduvai Gorge where they were first found. Early Oldowan tools are estimated to date back to about 2.5 million years ago and consist of crudely flaked chopper cores. These chunky flakes were produced by direct percussion using a suitable hammer stone, something like a piece of granite. The hammer stone would then have been used to touch up the initial flake and produce a finer cutting edge. It is possible that development of the earliest tool-making techniques predated Homo habilis, which would make some of the australopithecines the first tool makers.
The simple tools made by Homo habilis included those with sharp edges for cutting through hide to reach the meat of prey animals. Clubs, sharpened sticks, and edged stones could be used not only against prey, but as defensive weapons. Meat provided a compact source of protein and calories. The increase in meat consumption by Homo habilis required an adaptation of the digestive system, with a proportionally larger stomach and smaller intestine than the australopithecines.
The relatively large brain size of Homo habilis likely resulted in an extended period of helplessness for habilis infants. Since brain size at birth is restricted by the size of the pelvic opening, there would need to be a lengthy childhood during which the habilis brain grew to adult size. This extended childhood provided the time necessary to learn special skills such as toolmaking and coordinated hunting.
The stereoscopic vision that these early members of the Homo genus inherited from their arboreal ancestors suited their hunting endeavors. Like the leopards and saber-toothed cats they began to compete with, the bipedal hunters had forward-looking eyes that gave them superior depth perception.
Homo georgicus
Primitive human-like fossil skulls and jaws were found 1991-2001 by David Lordkipanidze, a Georgian anthropologist, at Dmanisi, in Georgia. In size and morphology, the Dmanisi skulls were similar to African Homo habilis. Simple stone tools were found at the site, near the Caucasus Mountains, and a partial skeleton was unearthed there in 2001.
The basalt tools found at the Dmanisi site were chipped in two directions to form coarse implements capable of cutting, chopping and scraping. The fossil remains were of beings that had a brain volume of 600 to 780 cc and were about 4 feet, 10 inches tall. The fossils were dated as 1.8 million years old. The type was assigned the designation Homo georgicus.
Georgicus was primitive in its head and upper body. The morphology of the shoulder blades and collar bones suggests that the upper body and arms of Homo georgicus were less human-like than Homo erectus. Georgicus had relatively advanced spines and lower limbs and was capable of running.
The environment at Dmanisi 1.8 million years ago was savannah-like, temperate, with nearby lakes and mountains. The georgicus fossils are the most primitive human-like remains found outside Africa. They appear to be from a form of Homo habilis or transitional between Homo habilis and Homo erectus, and suggest that a significant portion of early human evolution took place in the Caucasus.
Homo erectus
In 1891, Eugene Dubois, a Dutch anatomist, discovered at Trinil, on the banks of the Bengawan Solo River in eastern Java, the partial fossil remains of a creature that he thought was between apes and man. In the 1920’s and 1930’s, Davidson Black and Franz Weidenreich documented fossil remains found in China of what became known as Peking Man. The Java and Peking fossils were later recognized as belonging to a species, Homo erectus, that first appeared about 2 million years ago. Erectus had a long, low-browed skull, with prominent ridges above the eyes.
Homo erectus (upright man) had a brain size of about 900 cc in early specimens and 1200 cc in individuals after about 800,000 years ago. The larger brain size of Homo erectus began to approach modern man’s 1350 to 1450 cc. The height of Homo erectus ranged from 5 feet to 6 feet, and its weight from 90 to 160 lb. The preserved footprints of an early Homo erectus that lived 1.5 million years ago indicate an upright posture. Its leg and hip structure was well adapted for running.
Running was an adaptation with an extremely high payoff for a predator species. It allowed a rapid dash immediately preceding a kill, and the less quick sustained pursuit of prey. Valuable as it was, the capacity for running posed evolutionary difficulties. Apes have an abundant hair covering that provides insulation, useful for maintaining body temperature within narrow limits. For the new predator hominids, hairy bodies would present a problem during the all-important chase. The extreme effort of running generated large amounts of caloric energy, and thick body hair would work against heat dissipation and lead to overheating.
There would be intense selection pressure to reduce overheating. Effective cooling would make the difference between a successful kill followed by a rich meal, and debilitating hunger. A complementary evolutionary adaptation, enhanced evaporative cooling, changed the appearance of the human lineage. Bare skin was achieved by dispensing with thick body hair, and an increase in the number of sweat glands supplied the evaporating fluid. In early humans, body hair became much shorter and thinner than in the australopithecines. The development of a subcutaneous layer of fat helped to keep the body warm when at rest.
Like Homo habilis, erectus had hands with the ability to oppose or turn back the thumb against the other four fingers, enabling holding objects with a fine grip. An extended childhood allowed Homo erectus time for learning skills necessary for survival, and possibly also for learning a rudimentary form of speech.
With one exception, all living primates have the larynx positioned close to the throat, in a way that permits swallowing and breathing at the same time. The one exception is humans, which have the larynx positioned low in the throat. This arrangement does not allow humans to swallow and breathe at the same time, and accidental attempts to do so lead to choking. The peculiar configuration of the human larynx has the unique property of greatly increasing the range of sounds that can be produced in the pharyngeal space. It is necessary for speech.
The human neck and the lower part of the head are configured to work with the vocal tract. Analysis of the shape of the base of Homo erectus skulls show a flexed pattern, rather than the flat shape common to all primates except humans, suggesting erectus had a limited capability for speech. Australopithecine skulls have the typical flat bottom found in other primates. The skull fragments that have been found for African Homo habilis do not include the cranium base necessary for this type of analysis.
Erectus was a hunter-gatherer. Its diet included bird eggs, small game, shellfish, fruits, nuts, and large game, such as antelope. Hunting of large game was facilitated by the development of specialized tools of a type named Acheulean, such as hand axes, cleavers, picks and spears.
Francesco Berna and Paul Goldberg, of Boston University, and Michael Chazan, of the University of Toronto, reported in 2012 that they had found archaeological evidence of the use of fire by early humans about one million years ago. The evidence was of wood ash and burnt bone dug from the floor at Wonderwerk Cave, in the Kuruman Hills, about 200 miles west of Johannesburg, South Africa. Particles of ash found at the site indicate the presence of materials often used to start or promote fires, such as twigs and leaves. Stone tools found at the Wonderwerk location are of the Acheulean type usually associated with Homo erectus. Fire facilitated the preparation and preservation of meat, provided light in the darkness, and helped early man keep warm in cold weather.
The Acheulean stone tools associated with Homo erectus derive their name from a set of stone tools found at Saint Acheul in France. The tools originally found at Saint Acheul date from about 300,000 years ago, however, the earliest evidence of Acheulean tool technology is from about 1.7 million years ago. The classic example of an Acheulean stone tool is the hand axe.
It seems likely that Homo erectus started producing Oldowan style tools first, and then gradually developed Acheulean methods. Acheulean technologies are the first examples of bifaces, stone tools that are flaked on both sides. This requires rather more skill, and a many more strikes per tool than Oldowan. Acheulean tools were more symmetrical, and being thinner than single-faced choppers they were superior for cutting and slicing meat and hide.
Homo erectus hunters were able to carry out attacks on groups of large and dangerous animals. Communication between hunters would facilitate their coordinated action, which required the capacity to establish objectives, plan activity sequences, and assign individual roles. Gestures and sounds would be used to communicate, and the ability to assign explicit meanings to specific sounds provided the incentive for developing a set of shared significant vocalizations.
With a large brain, efficient bipedal stride, tools that included effective weapons, and an adventurous spirit inherited from risk-taking ancestors, Homo erectus ventured out of Africa about 1,800,000 years ago. Erectus became a proficient hunter of large game, such as mammoth and bison. Fossil remains of Homo erectus have been found from Africa to Europe, China, and Java. East Africa and Java specimens have been estimated as being from about 1.8 million years ago. Eastern Europe and Peking erectus remains are estimated to be from about 1.75 million and 500,000 years ago, respectively.
Eugene Dubois collected many fossils during his 1891 Java expedition. Among these were Homo erectus and various mammal and fish bones and over a hundred clam shells from the Trinil site. The fossils are entrusted to the Naturalis Center in Leiden, the Netherlands. In 2007, a graduate student photographed the shell collection and noticed striking zigzag scratches on the surface of a freshwater mussel shell. The discovery prompted an in-depth study of the Dubois shell collection. In 2014, Josephine Joordens and others at Leiden University published a paper in Nature concluding the shell engravings were made by a Homo erectus individual about 500,000 years ago.
The Joordens team found evidence of freshwater shellfish consumption by Homo erectus. About a third of the Trinil fossil shells had a small hole at the location of the anterior abductor muscle. The hole, probably made with a shark tooth, was used to damage the muscle and open the shell. These findings imply a fine grip and the use by Homo erectus of a tool like a shark tooth to both pierce the shell and engrave a systematic geometric pattern. The findings indicate symbolic actions by Homo erectus.
The remains of the earliest garments have not been found, but as Homo erectus moved into colder Europe and central Asia it must have started wearing animal skins to keep warm and as protection from the elements. During the Pleistocene epoch, starting about 1.8 million years ago, Earth’s climate underwent repeated fluctuations from warm to cold to warm. In the cold phases the glaciers expanded, formed vast ice sheets and caused a drop in sea levels, in the warmer interglacial phases these effects were reversed. These slow fluctuations, lasting thousands of years, were superimposed on a continued global cooling trend. Temperature change records determined from Antarctic ice core samples indicate that about 500,000 years ago average temperatures were about 7 deg F lower than at present. There is evidence that erectus wore animal hides at least as far back as 650,000 years ago. The stone cutting edges and scrapers in the erectus tool kit would have been used to make the garments from the hide and fur of prey animals.
Homo erectus disappeared from the fossil record about 400,000 years ago, but a dwarf variety, Homo floresiensis, persisted in isolation in the island of Flores, east of Java, until about 13,000 years ago.
Homo heidelbergensis
In 1907, Daniel Hartmann found a peculiar jaw bone in a quarry where he worked near Mauer, a village in the vicinity of Heidelberg in southwestern Germany. Hartmann took the jaw to Otto Schoetensack, who had organized an archaeological dig at the site. Schoetensack realized the jaw belonged to an extinct hominid that he eventually named Homo heidelbergensis. Later finds of Homo heidelbergensis fossils came from throughout Europe and were dated at from about 530,000 to 120,000 years ago. Similar fossils have also been found in China, and in Africa at Kabwe, in Zambia, and Bodo, in Ethiopia.
Homo erectus was likely ancestral to Homo heidelbergensis. Homo heidelbergensis had a taller build. Heidelbergensis also had a larger brain, about 1200 cc in volume, with larger frontal and parietal lobes, and its cranial bones were less thick. Heidelbergensis had a more rounded skull than erectus, but retained a robust mandible and facial structure. Males were on average about 5 ft, 9 in tall and weighed about 136 lb. However, the height of some heidelbergensis individuals has been estimated as up to 6 ft. Female height averaged 5 ft, 2 in and average weight was 112 lb.
Fossil remains of heidelbergensis are often associated with Acheulean stone tools like those used by erectus, but more precisely made, more symmetric and better trimmed.
In 1964, Francisco Jorda found hominid fossil remains and stone tools at the cave Sima de los Huesos, in the Atapuerca mountains in Spain. The site was further explored by others, in particular in 1978-1990 by a team led by Emiliano Aguirre, a Jesuit paleonlologist with the Complutense de Madrid University. The fossils, found in various locations and ground layers, were of a range of extinct hominid types as well as modern humans. Many of the fossil bones were of Homo heidelbergensis. Animal fossils were also found (ancient predators and bear, bison, and sheep). The Atapuerca digs indicate the caves and nearby sites were home to Homo heidelbergensis for hundreds of thousands of years, and later by Homo neanderthalensis and Homo sapiens. The extended range of occupation suggests the possibility of contact between the different hominid types that inhabited the area.
Homo heidelbergensis was the apex predator throughout its range. Heidelbergensis hunted deer, mammoth, horses, and other medium and large game. In 1995, Hartmut Thieme found sharp-pointed wooden spears and stone tools at Schoninger, Germany, alongside remains of horses butchered by heidelbergensis. The Schoninger artifacts date from about 400,000 years ago.
Archaeological evidence indicates heidelbergensis built shelters out of wood and stone and used hearths for cooking and heating at their campsites. Homo heidelbergensis is likely ancestral to Homo neanderthalensis in Europe and to Homo sapiens in Africa. Heidelbergensis is sometimes referred to as archaic human.
Homo neanderthalensis
In 1856, miners discovered a partial skull, some ribs, arm, and shoulder bones, thigh bones, and part of a pelvic bone at Feldhoffer in the Neander Valley in Germany. They took the bones to naturalist Johann Karl Fuhlrott, who was the first to study them. The bones were human-like, but were easily identifiable as not from a modern man. The Neander Valley find occurred before the publication of Charles Darwin’s Origin of Species in 1859, at a time when concepts of the evolution and extinction of species were not well developed. It was later determined that skullcaps discovered in Belgium in 1829 and in Gibraltar in 1848 belonged to the same species as the Neander Valley bones. The term Neanderthal (Neander Valley) Man was first used by William King in 1863.
Neanderthals first appeared about 150,000 years ago. They were well adapted to the cold climates that prevailed throughout their range in Europe and northwestern Asia. Their remains have been found from Spain, France and Germany, to northern Egypt, Palestine, and Kazakhstan. They were powerfully built, and had barrel-shaped rib cages, relatively short limbs, and stocky builds, with thicker bones than modern humans. Their height averaged about 5 feet, 5 inches for males and five feet, 1 inch for females. Neanderthal brains averaged about 1410 cc in volume, similar to modern men. They had low, elongated skulls, with prominent brow ridges, large projecting noses, and receding chins. They had a descended larynx, a hyoid bone similar to modern humans, implying some ability of speech, and probably developed a primitive language.
There is archaeological evidence that Neanderthals routinely controlled fire, often made use of caves and rock shelters, and built temporary shelters from wood posts and brush or leaf coverings. They practiced ritual burials and had a degree of culture and social organization. Neanderthals made tools of a type called Mousterian, characterized by the preliminary shaping of a stone from which the final tool was struck off. The process of toolmaking was standardized into stages (core, rough blank, final shaped tool).
Mousterian stone tool technology dates from about 300,000 years ago. It was named after Le Moustier, a rock shelter in the Dordogne region of France, where tools of the type were discovered in the 1860’s. The technology is typically associated with Homo neanderthalensis, although the dating of the earliest samples suggest it is possible that it was first developed by Homo heidelbergensis. Mousterian techniques incorporated a set of improvements designed to maximize the number of tools produced from a piece of flint. Mousterian tools were produced from flakes removed from the cores, not the cores themselves.
One Mousterian technique was the blade core technology, where multiple long blades were struck from one core. Core technologies required more accurate striking of the core, for example, by using indirect percussion (a hammer stone hitting a chisel made of wood, bone or antler) to improve precision. A second Mousterian technique was flake core technology (Levallois flakes), which produced rather regular, flattened flakes rather than blades. A key feature of these flakes is that both sides are convex, so that the tool is both strong and sharp.
Neanderthals began to be replaced by modern humans around 45,000 years ago. The newer type and Neanderthal coexisted for thousands of years, but the Neanderthals were less effective in adapting to environmental changes accompanying the thinning out of European forests. It is theorized that Neanderthals used primarily a stalking hunting style to bring down large animals. They would use the cover provided by forest vegetation to approach their prey unnoticed, and then rush ahead and impale the animals with sturdy thrusting spears. If the kill was not made immediately, the Neanderthals would use their powerful builds to force the injured animal down or hold it until other hunters finished it off. This hunting style became less effective as the forests receded and cover was less often available. During the period from 30,000 to 20,000 years ago, Europe and northwestern Asia experienced the coldest temperatures of the last glacial period, about 15 deg F below current temperatures. Tundra and glaciers covered much of the land. The range of Neanderthals was gradually reduced to places like Spain, Croatia, and the Crimea.
The fossil records of Neanderthals stop at about 25,000 years ago. Available archaeological and genetic data indicate that modern humans did not interbreed significantly with Neanderthals and are not descended from them.
Homo sapiens
Anatomically modern humans, Homo sapiens, first appeared in sub-Saharan Africa about 120,000 years ago. Compared to earlier forms, they were tall and slender, with longer limbs and a narrower and shorter torso. Their face was flatter, the brow ridges much reduced, the forehead higher, the chin more prominent, and the internal nasal cavities smaller. The skull bones, less thick than in Homo heidelbergensis, enclosed a brain about 1350 to 1450 cc.
In 1997, Tim White, an American paleoanthropologist, discovered the remains of an early form of anatomically modern human in the Afar region of Ethiopia. The remains were dated to about 157,000 years ago. The Afar specimens have some archaic features not found in modern humans, and may represent a transitional form from Homo heidelbergensis.
Fossil bones more closely resembling those of modern humans were found at Omo, Ethiopia, in 1967. The Omo specimens are not accurately dated, but are estimated to be of from 60,000 to 130,000 years old. The Omo skull has a short, broad face, a high forehead, dentition like modern man, and a 1400 cc brain size.
Droughts affecting eastern Africa provided a harsh environment that put the relatively small Homo sapiens population under intense evolutionary stress. These earliest peoples lived on the edge of extinction until the climate improved about 90,000 years ago. There were population shifts throughout sub-Saharan Africa, but sapiens remained in the African continent until about 70,000 years ago.
Early Homo sapiens used cutting tools, points, and scrapers of the type called Mousterian. They probably used animal hides to make clothes, shoes, sacks, and tents. Homo sapiens moved out of Africa into Arabia and southern Asia, and then into India and southeast Asia. By 50,000 years ago they had reached Australia.
In the period between 50,000 and 40,000 years ago, groups of Homo sapiens moved across Asia and into Europe. Homo sapiens populations moved northward into southeastern Europe from the Middle East. These expanding Homo sapiens populations must have met bands of Homo neanderthalensis. From southwestern Asia, Homo sapiens expanded to the north and east across Asia. Between 40,000 and 30,000 years ago anatomically modern man expanded into western Europe and northeastern Asia. Population movements sometimes needed to overcome physical barriers that once surmounted were unlikely to be crossed again. These barriers at times resulted in the practical isolation of populations. Over time, the isolated populations tended to become differentiated.
In 1868, Louis Lartet, a geologist, discovered bones of several ancient humans in a cavity in the Cro-Magnon rock shelter at Dordogne, in France. The skeletal characteristics were very similar to those of modern humans, only slightly more robust, but were clearly prehistoric. This type of Homo sapiens was named Cro-Magnon Man, and similar specimens were later found at various locations in Europe and in the Middle East. Cro-Magnon lived in caves and in huts made with tree branches, stones, clay, and animal bones and skins. They had flint and bone tools, and made beads and bracelets. Cro-Magnon remains and artifacts have been dated from about 45,000 to about 10,000 years ago.
Archaeological evidence links Homo sapiens to a technology called Aurignacian beginning about 45,000 years ago. The Aurignacian industry was characterized by flint tools that included fine blades and bladelets struck from prepared cores rather than using crude flakes. Aurignacian stone tools were typically more standardized than Mousterian or Acheulean counterparts. These tools are often associated with Homo sapiens fossils. The term Aurignacian derives from Aurignac, a region near Toulouse in southwestern France, where Edouard Lartet discovered prehistoric human remains in 1860. A significant aspect of this technology is its association with art forms. Aurignacian techniques are associated with bone, ivory and antler artifacts, including figurines and ornaments.
Aurignacian artifacts, dating from 45,000 to 30,000 years ago, have been found in Europe, Asia and Africa. Aurignacian tools were found at Ksar Akil, a rock shelter near Beirut, Lebanon, at the same level as the remains of an eight year-old anatomically modern human that lived about 43,000 years ago.
There was a marked change in Homo sapiens activities about 35,000 years ago. The archaeological record indicates that carvings and new, more precisely made tool types began to be produced. The earliest evidence of the use of tallies and numbers dates from this period. Around 30,000 years ago, a method was developed for making fire-hardened clay figures, and a new type of hunting weapon was developed, the spear thrower. It is likely that the makers of these objects had a well-developed language capability.
The spear thrower extended the range at which the largest animals could be killed. A short shaft with a cupped end was used in conjunction with a thin stone-tipped spear. In use, the base of the spear was positioned against the cup of the thrower, and the spear was thrown by swinging the hand-held thrower. The weapon had an effective range of 100 to 300 feet. Chemical analysis of Cro-Magnon remains, measuring the occurrence of element isotopes, indicate that their diet included both herbivores and aquatic animals. The practice of catching and consuming fish added to the variety of available food sources, and would have enhanced the probability of surviving periods of poor prospects for large game hunting. These and other signs of cultural and technological evolution coincide with the expansion of Homo sapiens in Western Europe, and with the final decline of Homo neanderthalensis.
String, made by twisting together plant fibers, was invented sometime before 27,000 years ago. Some type of bast likely provided the fibers. String was also made from animal parts such as wool, gut and sinew. Thread had a number of applications, one being its use to fasten pieces of hide together to make fitted garments. Another use of thread was to fasten stone or bone projectile points to wood shafts.
Small implements such as needles suggest Cro-Magnons at some point learned how to weave clothing. Archaeological evidence of fabric, surfaces made out of thread, appears in the Middle East and Europe in the Paleolithic era. A carved ivory figurine made about 27,000 years ago, of a female human wearing a skirt of twisted string, was found in 1922 at Lespugue, France. Fiber fragments dating from about 15,000 B.C. have been found at the Lascaux caves in southwestern France. The first fabrics were likely made by stretching threads between parallel sticks and twisting neighboring threads around each other. Needles and hooks, made from bone and able to penetrate fabric and hides, were used to make clothes and shoes. Ivory needles, with drilled eyes, dating from about 30,000 years ago, have been found near Moscow, Russia. Similar ivory needles, from about 21,000 years ago, were found at Jouclas, France.
Cave wall paintings and bas relief carvings made by Homo sapiens date from about 20,000 B.C. More detailed cave wall paintings and carved objects, dating from 16,000 to 9,000 B.C., have been found in various sites in Europe. This Cro-Magnon art, including the depiction of specific individuals, suggests a level of self-awareness like in present-day humans.
The domestication of the gray wolf took place by 15,000 B.C., and led to the subsequent breeding of dogs as human helpers and companions. This was the first step taken by humans in the domestication of animals and plants.
Around 14,000 or 15,000 years ago, hunters in northeastern Asia crossed over to Alaska, probably following large game animals. At that time, and until 10,000 years ago, great glaciers covered most of northern North America. Although in the American continent there were no Neanderthals to contend with, and bison and other large game were abundant, the human hunters had to deal with saber-toothed tigers, bears, and dire wolves. Early human occupation was for the most part originally limited to the western coast and the southwest of North America. Over a few thousands of years, nomadic groups in North America moved south, proceeding to Central and South America.
A global warming trend that began about 20,000 years ago brought average worldwide temperatures to about the current level by 10,000 years ago, the start of the Holocene, the current interglacial epoch. The Holocene saw the further migration and differentiation of modern humans. Forms of rye and barley were put to domestic use in the Middle East by 9,000 B.C., and sheep were first domesticated by about that time.
The Line of Human Descent
What is the line of descent of humans? The Bible’s account of the creation of Adam and Eve has the first humans formed at God’s will. Thus, a literal interpretation of Scripture suggests that there is no line of descent. But what can science tell us? Anthropology and paleoanthropology are among the most controversial scientific disciplines. The incomplete and often fragmentary information available often gives rise to speculation in order to fill in the gaps in the factual record. Theories abound about human origins, and they are often revised to accommodate new facts. Bearing in mind that speculation is not the same as factual knowledge, what can be inferred from the accumulated information physics, chemistry, biology, archaeology and anthropology make available? It turns out many inferences about the human line of descent can be made from scientific records, with varying degrees of certainty.
One inference that can be made with a high degree of certainty is that all living organisms on Earth, past and present, are somehow related. This inference is based on the observation that all organisms whose genetic material has been tested share the same basic genetic code, made up of the chemicals deoxyribose sugar, phosphate, adenine, cytosine, guanine, and thymine. The genetic material in all lifeforms on this planet shares the same ratio of adenine to thymine and guanine to cytosine. The arrangement of molecules in deoxyribonucleic acid (DNA), carrying information for building and maintaining organisms, is common to all lifeforms. DNA’s alternating deoxyribose and phosphate molecules form the twisted strands of the DNA helix, and complementary pairs of adenine and thymine, and cytosine and guanine, link the two strands, like rungs on a ladder.
Biology and taxonomy classify organisms, by consensus of the scientific community, into ranks of kingdoms, phylums, classes, orders, families, genuses, and species. A coup d’oeil lets us place humans in the animal kingdom, phylum Chordata, class Mammalia, order Primates, also with a high degree of certainty. It can be inferred with a high degree of confidence that humans are descended from a family of extinct apes that lived in Africa about seven million years ago. Further inferences about the human line of descent can be made, with less certainty.
Among the genetic differences that exist between humans and all the contemporaneous apes, there is one that stands out most obviously. Humans have 46 chromosomes, while living great apes have 48. The occurrence of this major mutation separates the human line from the ancestral line of orangutans, gorillas and chimpanzees. A genetic change of this nature signals a major mutation, absent in many closely-related species. For example, domestic cats, cheetahs, jaguars, tigers, and lions all have 38 chromosomes. Dogs, African wild dogs, wolves, coyotes, and jackals all have 78 chromosomes. Rhesus monkeys, an Old World (catarrhini) species, have 48 chromosomes, like orangutans, gorillas and chimpanzees. The more primitive Capuchin monkeys, a New World (platyrrhini) species, have 54 chromosomes.
It is not known when the mutation that led to the number of human chromosomes being reduced to 46 took place, but it seems to have occurred not earlier than the split, about eight million years ago, between the line of human descent and the ancestral line of gorillas and chimpanzees.
There are marked differences between the skeletons of humans and existing apes (gorillas, chimpanzees and orangutans). The human cranium, spine, arms, legs, and, in particular, the bones of the foot, show adaptations that are absent from current ape species. Humans lack a bone, the baculum (penis bone), that is present in most primates, including current ape species.
The fossil record indicates that bipedalism first appeared in transitional hominids like Orrorin tugenensis and Ardipithecus ramidus. Bipedalism is a distinctive characteristic of humans, and it is reasonable to infer that these species, or similar unidentified ones living seven to four million years ago, are in the human line of descent.
The dentition of Australopithecus anamensis and anatomical similarities link it to the earlier Ardipithecus ramidus and suggests it was a descendant of ramidus.
The chronology of fossil records and physical similarity can guide reasonable assumptions about the line of descent of identified species. Australopithecus anamensis leg bones indicate bipedalism, and afarensis and africanus clearly show bipedalism. All the australopithecine fossils have been found in Africa, indicating that their development took place within a limited range. The more recent species show increasingly advanced features. Following a parsimonious approach, without postulating hypothetical undiscovered species, identifies the likely successor of anamensis as Australopithecus afarensis, and Australopithecus africanus as the descendant of afarensis.
Sometime before two million years ago, the australopithecines came under great evolutionary stress, possibly caused by climate changes that led to open fields replacing fruit-bearing trees in parts of sub-Saharan Africa. There is some uncertainty about what happened next. One australopithecine branch, Australopithecus robustus, with adaptations to a diet of tough plant foods, appears to be an evolutionary dead end. The other branch likely led to Homo habilis, the first species of the genus Homo, a tool maker and a hunter, with a significantly larger brain than the australopithecines.
Late forms of Homo habilis, with improved bipedalism that made them capable runners, were the first in the human line to migrate out of Africa, into the Middle East and the Caucasus region.
The successor of Homo habilis was likely Homo erectus, taller and with a larger brain than habilis. Homo erectus expanded to Asia and Europe. The fossil record shows the gradual evolution of Homo erectus, with increasing brain size and development of more finely made tools.
Later forms of Homo erectus are similar to the earliest of Homo heidelbergensis, supporting the conjecture that erectus was the ancestor of heidelbergensis. Homo heidelbergensis fossils have larger brains and other more advanced characteristics than Homo erectus.
Homo neanderthalensis specimens have similarities with Homo heidelbergensis and were likely descended from them. Neanderthalensis was anatomically adapted to life in colder climates. Its hunting tools and techniques made neanderthalensis the apex predator in northern forests. Neanderthalensis possibly became overly specialized and found it difficult to adapt to climate changes that thinned out the forests and altered the availability of large prey.
The earliest Homo sapiens had characteristics, like a thrust-forward chin, a lighter bone structure, and a less-elongated skull, different than Homo heidelbergensis or Homo neanderthalensis. Homo neanderthalensis had been around for tens of thousands of years when Homo sapiens first appeared. Before the climate changes that caused northern forests to recede, early Homo sapiens occupied some locations in the Middle East where Homo neanderthalensis later appeared, suggesting that under the conditions then prevailing early Homo sapiens did not outcompete the Neanderthals. Later, when Homo sapiens moved into Europe, it had acquired more advanced tools and possibly some evolutionary change, perhaps an improved language capability or greater intelligence, that gave the species an advantage over neanderthalensis. The two species occupied overlapping territories for thousands of years in the Middle East and in Eastern Europe, but after about 40,000 years ago Homo sapiens began to replace neanderthalensis.
It is possible that the sapiens body plan, with a more slender build, smaller torso, and longer legs, gave an advantage over neanderthalensis in the harsh environment of 40,000-25,000 years ago. Neanderthals were more heavily built, with a stronger musculature and thicker bones, but sapiens had a longer stride, would have been swifter, and may have required a smaller caloric intake to survive.
It appears that Homo neanderthalensis was an evolutionary dead end, unable to endure the combined stress of habitat changes and a more adaptable competing species. Anthropological and genetic data indicate that modern humans did not descend from Neanderthals. The most straightforward conclusion is that Homo sapiens descended from Homo heidelbergensis.
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