Human ancestral species first diverged from ancestors of the apes around 6 million years ago in Africa. First the species of the genus Australopithecus, and then later species of the genus Homo, emerged during successive periods in which the rate of speciation (divergence into new species) increased. These waves of speciation occurred not just among hominids but among many other species and coincided with an important series of climate changes that began around 6 million years ago, in which the prevailing warm and moist tropical to subtropical climate of Africa was interrupted for periods several million years long by cooler and drier weather. Fossils documenting the divergence of apes and hominids are lacking. But evidence provided by studies of genetic differences between humans and apes, showing how long ago they shared a common ancestor, suggests a divergence in the same time frame as the climate shifts in Africa.
Cooler and drier conditions created openings in the dense tropical forests, creating a mixed habitat with different zones of forest and of open savannah or grasslands. This led to a divergence between forest-adapted species ancestral to the apes and species ancestral to hominids. The australopithecines were well adapted to open terrain because they could walk upright, roam around searching for food, and, if threatened, retreat quickly to the forest shelter carrying the food they had found.
In the absence of fossil evidence this scenario for the original divergence of apes and hominids is speculative, but for the period of the next wave of speciation, around 3 to 2 million years ago, again coinciding with a return to cooler and drier conditions, fossils of hominids and of many other species they associated with have been found. They show that many forest-dwelling species of bovids such as antelopes became extinct at this time, and others suited to open habitats evolved. At the same time the forest-dwelling hominid genus Australopithecus diverged into species of the genus Paranthropus (although this genus is not accepted by all paleontologists), which, as did their immediate ancestors and the apes, continued a herbivorous forest existence, and into species of the genus Homo, which exploited the new habitats as aggressive, omnivorous scavengers and possibly hunters. The open-ranging bovids began to disperse and migrate over wide areas, probably as a result of the lower density of nutrients in the grasslands that resulted from the cooler, drier conditions. The hominids of Homo with their larger size, longer legs, and a pelvic structure allowing a ranging, bipedal gate, were adapted to follow the herds and may have dispersed with them.
GENUS AUSTRALOPITHECUS_
The primary characteristic that distinguished the australop-ithecines from other primates was the ability to walk fully upright. This genus was first known from the discovery in 1924 of the skull of a juvenile, nicknamed the “Taung baby” after the site in South Africa near which it was found. Australopithecus means “southern ape.” The position of the foramen magnum, the opening in the skull through which the spinal cord attaches to the brain, of this specimen shows that it could walk upright. It was named Australopithecus africanus and lived from before 3 million years ago to approximately 2 million years ago. The date of evolution of Australopithecus was pushed back in time with the discovery of the remarkably complete skeleton of a female (nicknamed Lucy) in 1974 near Hadar in Ethiopia. Subsequently named Australopithecus afarensis, this species is believed to have lived from approximately 4 to 2.7 million years ago along the northern Rift valley of east Africa. An earlier species of Australopithecus, tentatively called Australopithecus anamensis, was found in 1994 by Mary Leakey, who with her husband, Louis Leakey, pioneered the search for human ancestors in Africa. This species lived 4.2 million years ago.
Australopithecines did not have large brains; their skull and facial characteristics were more like those of chimpanzees, as was their size: Lucy was only three feet eight inches tall. Males were considerably larger; this sexual dimorphism (large differences between males and females) is more typical of apes than of humans. Their arms were longer relative to their legs than is the case with humans. This proportion is quantified by using the humerofemoral index, the ratio of the length of the humerus, the upper arm bone, to that of the femur, the upper leg bone. An index of 100 means both are the same length; modern humans have an index of 70, meaning our upper arms are 70 percent of the length of our upper legs, whereas chimpanzees have an index of nearly 100, as did australop-ithecines. Thus australopithecines are classified as “hominid apes,” that is, apes closely related to humans.
A number of other species of Australopithecus are known and have been grouped according to two distinct body types, gracile and robust. Gracile australopithecines have lighter bones, especially in the face, than robust species, such as Australopithecus boisei, which have powerful jaws and teeth adapted to crushing tough foodstuffs, probably mostly plant foods, and a ridge running from front to back on the top of their skull that served to anchor powerful jaw muscles. Robust species are classified alternately as belonging to the genus Paranthropus or as being part of Australopithecus. It is not clear as yet which lineage of Australopithecus is directly ancestral to Homo; some researchers favor Australopithecus africanus and others Australopithecus afarensis. In general the fossil record of early hominids is still so sparse that relationships between lineages are difficult to establish.
GENUS HOMO_
The earliest members of the genus Homo are Homo habilis (handy man) and Homo rudolfensis. Homo habilis, first reported in 1964 on the basis of fossils discovered by Louis Leakey and his colleagues at Olduvai Gorge in east Africa, though having a brain size considerably smaller than that of modern humans, was claimed as human on the basis of numerous finds of stone tools in the same geologic horizon (layer) as the jaw and cranial specimens. This species lived 2 million years ago, as did Homo rudolfensis. These hominids were contemporaries of Australopithecus africanus and other australopithecines. There is currently debate as to which hominid species evolved into Homo ergaster, which is considered the probable ancestor of modern humans. It was probably Homo ergaster that, 1.6 million years ago, developed an advanced type of stone working called the Achulean stone tool industry.
With Homo ergaster there occurred a dramatic increase in brain size and typically human behavior such as the use of fire and hunting. Homo ergaster was tall and thin, probably an adaptation to activity in hot weather, and had narrow hips relative to adult brain size, meaning that their young were born small and helpless. Apes give birth to infants whose brain’s weight is 50% of the weight of an adult brain, whereas modern human babies are born with a brain only 25% the adult size. Homo ergaster could not have given birth to an infant with a brain more than 30% its adult size. The long childhood of humans is considered a crucial element in fostering intelligence, learning, and socialization.
HOMINIDS LEAVE AFRICA_
The first early hominid fossil found outside Africa is that of a skullcap found near the Solo River in Java; it is called Java Man. In the 1950s paleontologists decided that this species, of whom the remains of 40 individuals have been recovered, was the same as that of fossils from caves near Zhoukoudian, China, discovered in the 1930s, which had been called Peking Man. The two were combined into a species called Homo erec-tus. Dating of Homo erectus finds has been problematic; once this species was thought to be no more than 1.1 million years old; new dating methods have now pushed back the age of some fossils to 1.6 million years. Claims of dates of as much as 1.8 and 1.9 million years ago have been made. These last are provisional as yet and 1.6 million years old is so far the most reliable date. There is also some disagreement as to whether some early specimens of Homo ergaster found in Kenya and dated to as much as 1.8 million years old are actually Homo erectus, though the consensus is against this view. In general human paleontology remains a volatile field; again, because so few specimens exist, a single find can have a dramatic effect in realigning the human family tree.
If the claims of dates of 1.8 million years old for Homo erectus in Asia are discounted, the earliest hominid find out of Africa becomes that of two skulls found in 1999 in Dmanisi, in the Republic of Georgia, whose date of 1.7 million years old is considered reliable since it is based on dating of volcanic sediments contemporary with the fossils. These skulls are closer to Homo ergaster than to Asian Homo erectus. More than a thousand stone tools were found with the skulls; they are important because they were made in the more primitive Olduwan style used by Homo habilis of Africa, showing that Homo’s migration from Africa was not a result of the development of better tools. This find also pushes back the date of the arrival of Homo in Europe, once thought to have been first accomplished by a more advanced human, Homo heidelbergensis.
HOMO HEIDELBERGENSIS_
More fossil evidence exists for the precursors of the Neanderthals, Homo heidelbergensis, than for earlier hominids. The earliest traces of Homo heidelbergensis are a lower jaw from a site in Mauer, Germany, and part of a leg bone from Boxgrove, England, both about 500,000 years old. More complete specimens date from 400,000 to 200,000 years old. Homo heidelbergensis had a large brain and a robust build, pointing to the body type of the Neanderthals. The species is associated with some of the first evidence (400,000 years old) of shelter building, saplings stuck into the ground in a circle with their tops drawn together, and the use of an interior hearth. At Schoeningen, Germany, a collection of carefully shaped six-foot-long wooden throwing spears that were found (with their center of balance to the front) date to about 400,000 years ago. This represents a “great leap forward” in technological innovation once thought exclusive to Homo sapiens; throwing spears allowed hunters to attack prey from a distance, a far less dangerous method than using earlier thrusting spears.
Little evidence has been found from this period of settlement in any one place for an appreciable length of time. More common are sites with a low-density scatter of chipped stone tools and animal bones showing marks of butchering. The stone used for making the tools is always close by, about 10 miles away at most, and could easily have been gathered and taken to the butchering site within a day. The normal sequence, then, was to hunt and kill an animal, then scout around for a suitable stone to make tools for butchering. In areas without flint or chert, which can be made into very sharp cutters, people made do with whatever they found, including coarse-grained basalts and quartzites. The sequence from making a kill to finding stones and making tools, then finally butchering could have taken as little as a few hours or even minutes. This implies a lack of long-range planning and the ability to project ideas very far into the future, even to carrying and reusing better flint and chert tools rather than simply discarding them after use.
World History
