Fossils representing the precursors of the primates may go back before the extinction of the dinosaurs, 65 million years ago. There are some primate-like teeth and bones found in Montana and Wyoming, dated from 60 to 65 maya. But the first undisputed primates appear about 55 maya. These early primates, despite the modern primate arboreal theme, may not themselves have been arboreal. As these primates continue to evolve, these basic traits prove a useful adaptive response to a more generalized life in the trees. Later the eastern and western hemispheres became completely separate, dividing the early primates into two geographical groups. The early new world primates were apparently replaced by more advanced monkey like primates that migrated from Africa to the Americas when the two continents were closer together by “island hopping” over a chain of volcanic islands or by rafting. Apes appear in the fossil record about 23 maya. With the evolution of larger bodies and larger brains, they became a successful group of primates.
Bipedalism was the first hominid feature to evolve millions of years before our big brains and flat faces. Each new fossil or new date or new interpretation of ancient environments changes the outlook slightly. Early hominid fossils found in eastern and southern Africa have long been linked to the Savannas that began expanding because of climate changes about 5 maya. It freed the forelimbs to carry things, including offspring and food. The vertical orientation helped cool the body by exposing a smaller surface area to the intense equatorial rays of the sun, and by placing more of the body above the ground to catch cooling air currents. It was very efficient and required less energy for long periods of steady walking.
Beginning with the first evidence of habitual bipedalism a little over 4 maya, the hominid fossil record becomes more complete and more complex. The hominid fossil record begins with an enigmatic set of specimens from Ethiopia dated at 4.4 maya. At present place in a separate genus, Ardipithecus, these fossils have been described as belonging to a forest dwelling, very ape-like biped. The earliest well established hominid fossils are placed in genus Australopithecus and are often divided into as many as three or four species. Their bones show full upright walking, but their faces are ape-like, their brain sizes approximate those of chimpanzees, their bodies average about 105 pounds, and their arms are long and heavily muscled. They were probably well adapted to both arboreal and terrestrial environments, and microscopic analysis of their teeth indicates a mixed vegetable diet of fruits and leaves. Early hominids made tools from stone as far back as 2.6 maya. Stone tools last for millions of years and provide evidence that were tools. Tools were used for such as: sharpening branching for crude spears or sticks for digging up roots, or cutting up plant materials for food or other purposes. It appears that stone tools allowed early homo to better exploit a source of food that was no doubt exploited before but would have entailed difficulty and danger - the scavenging of meat and bones from the carcasses of dead animals.
Evidently the adaptation of early homo proved so successful that hominid evolution seems to accelerate every 2 maya. Within about 800,000 years of the first evidence of stone tools in Africa, fossils of homo are found as far away as Georgia, China, and Java. Homo sapiens are seen as a 2 million year old species that displays a great deal of variation over time, but has always maintained enough to remain a single species.
Beginning about 1.8 maya, we find fossils representing what looks like a fairly sudden jump in our evolution. Body size is now within the modern human range and is essentially modern in shape. From the neck up, the bones reveal the retention of primitive features with one notable exception. Brain size has now evolved to an average of 960 ml and a maximum of 1250 ml. This is the Homo erectus stage named because when the first of these fossils were found in Java around the time of the 20th century, they were thought to be the first humans to walk upright. Fossils belonging to the Homo erectus stage are dated from 1.8 maya to perhaps as recently 100,000 years ago. Homo erectus members were expanding their range, they were coming into contact with the changeable environments of the ice age. This period was from 1.6 maya to around 10,000 years ago, when a decrease in the earths average temperature caused great sheets of ice, glaciers, to advance in the polar regions and out of higher elevations.
The brain size, and some other detailed features associated with modern humans, appears to have been achieved by some fossils from Spain dated at 780,000 years ago and from Tanzania and Ethiopia at 700,000 and 600,000 years ago. An early achievement of the stage dated to around 200, 000 years ago and appearing first in Africa, was a new tool marking technique. It’s called Levallois, or prepared-core, technique and essentially allows for the production of a number of predictably shaped flakes off a single core – historys’ first example of mass production. Although the Neanderthals are sometimes considered a separate species, we will here include them in Archaic Homo sapiens because of their modern size brains and retention of primitive cranial features. The bones of the Neanderthals, even the finger bones, were sturdier with heavier muscle markings than those of modern humans or for the matter, other archaics. They were stocky, muscular, powerful people, and these traits are even seen in Neanderthal children, so they are assumed to be the result of inheritance, not simply a hard working lifestyle. Based on features of fossils, as well as genetic analysis, some authorities feel Neanderthals are a separate species that made no contribution to the modern gene pool. According to them, the Neanderthal set of physical features died out when the species was replaced by more modern humans. Neanderthals were certainly a long lived group who were successful in adapting to some harsh and demanding climatic conditions, and who were clearly intelligent and sentient beings.
Beginning perhaps as early as 300,000 years ago, fossils with near modern features appear, earliest in Africa and later in other parts of the old world. We call these fossils anatomically modern because they lack some features characteristic of earlier hominids and possessive features common to humans today. Stone tools now show a great deal of craftsmanship. Some are so thin and delicate they must not have been made for a utilitarian purpose. There is clear evidence of big game hunting. And art makes an appearance in the form of carvings, and starting about 32,000 years ago, paintings in caves that rival art produced today.
A missing skull was discovered in England 40 years later proven to be a fraud. It was considered a missing link because it possessed traits that were a perfect mix between those of human and ape. Its cranium was the shape and size of a modern human and its lower jaw was decidedly apelike. For much of the history of evolutionary though, evolution was conceived of as a ladder or chain progressing from primitive to modern. We can recognize today that living species are not leftover primitive links on an evolutionary chain but rather contemporary products of evolution. A missing link in the traditional sense an intermediate between modern humans and modern apes simply does not exist. What does exist is a common ancestor of humans and our closest relatives the chimpanzees and bonobos, and we have reason to think that the common ancestor resembles a bonobo or chimp more than a modern human. This is because evolution happens to have taken place at a more rapid pace in hominids than in apes are still modern species.
