While the mass extinction ended the dinosaurs’ reign, it cleared the way for another group of animals to develop and prosper, the mammals. These animals differed from their reptilian ancestors in that they were warm-blooded, cared for their offspring, and even internally produced food for their young (i.e., milk). One such group, the primates, showed particular promise. These small, tree-dwelling creatures (akin to modern day lemurs) developed complex social interactions, emotions, and the ability to use sticks as primitive tools.
Recent fossil evidence of the species Darwinius masillae (Ida fossil) indicates that around 47 million years ago a branch of these early primates split off from their ancestors and developed into the anthropoids (now designated by the classification Haplorhini). This group became the great apes, chimpanzees, orangutans, and finally Homo sapiens sapiens (modern humans). The myriad of fossils uncovered in Africa show that Darwin clearly had it right, human beings did evolve from an ancient “monkey.” Many of the finer details are yet to be worked. However, decades of painstaking excavations, accompanied by meticulous bone fragment examinations, and comprehensive physiological modeling have allowed anthropologists for the first time to construct a fairly complete outline of this progression.
Anthropologists now believe that the seven-million-year-old fossil of Sahelanthropustchadensis (known as Toumai) represents the first human ancestor, for it exhibited a continuous brow ridge and short canine teeth–a trait shared only by the genus Homo. However, while one can debate whether or not any particular fossil is a direct link to modern humans or a distant cousin (for it now appears that a number of hominid variations coexisted at times, with overlaps of more advanced and archaic species), the chronological sequence of changes observed in these creatures directly lead to those traits that distinguish humanity from his Haplorhini brethren.
The first major step along this road occurred with just that, a step. The six million-year-old fossil of Orrorin Tugunensis (dubbed Millennium Man presumably due to its initial discovery in the year 2000) indicated that it walked upright. This represented a significant shift from our primate ancestors who resided primarily in the trees and was most likely a response to the changing African environment, from that of dense forest to forest patches surrounded by savannah. Walking upright allowed Millennium Man to use his arms and hands to carry more food further, most likely back to his mate, and would have presented a clear survival advantage. The fossil evidence from later forms, such as the 5.7 million-year-old Ardipithecus kadabba and 4.4 million-year-old Ardipithecus ramidus (nicknamed Ardi), continue to show evolution in the lower frame, resulting in more efficient bipedalism. However, these hominids still spent a considerable amount of time in the forest canopy and from the waist up looked more ape than human.
The next major advance occurred with the genus Australopithecus, which lived between four and two million years ago. From the hundreds of fossils discovered, anthropologists have identified seven distinct species–the most publicized being the fossil nicknamed “Lucy” (from the species Australopithecus Afarensis). Although Australopithecines’ brains were not much larger than those of Ardipithecus or chimpanzees (380-530 vs. 380 vs. 300 cm3), they did show a marked difference in structure from these earlier forms. First, their rounded shape more closely resembles that of a human brain. Second, cranial casts show that the lunate sulcus, a groove in the occipital lobe of the brain of primates, moved further toward the back of the skull. This would imply that the frontal lobe, that portion of the brain responsible for cognitive reasoning, occupied a proportionally larger section. Essentially, Australopithecus brains were being reapportioned toward thinking. Additionally, refinements to the hips and legs allowed these creatures to walk more efficiently and over greater distances. Coincident with this, the size of the hair follicles began to shrink over most of the areas of their body. This process would eventually lead to hominids losing the thick, matted fur coats donned by their ancestors, allowing them to better regulate body heat through the process of sweating. These creatures ably survived the slowly drying African landscape for more than two million years.
Then, around 2.5 million years ago, the environment of the African Rift Valley began to experience dramatic, cyclical swings. For the next 200,000 years, the climate fluctuated between drought and torrential rains. This changing landscape put pressures on all those who called this region home, including the various species of hominids; many would not survive. However, out of this turmoil would arise Homo habilis (skillful man), the first line to be considered true humans.
Analysis of Homo habilis remains shows that their brains continued to grow and their facial features softened. Although still small compared to a modern human, their brains showed about a 50% size increase compared to the Australopithecines. Moreover, a bulge in the section known as Broca’s area indicated that this hominid could have been capable of rudimentary speech. Another seemingly minor change would have a profound impact; a small bone in their thumb considerably broadened, allowing them to grasp objects with precision just as modern humans can. This physiological development relates to the most profound discovery associated with Homo habilis, as excavations sites of their remains are littered with crafted stone tools. These tools allowed them to scrape meat off bones and then crush the bones to extract the marrow. A meat diet provided the necessary protein to allow their brains to continue to grow.
Around 1.5 million years ago, archeologists note the arrival of a new genus, Homo ergaster. Home ergaster possessed a more recognizably human face, with a more rounded skull and projecting nose. They bore a muscular but essentially human build, with short arms and long legs (scientists estimate their height at 6 feet or more) and probably had no more hair than modern humans. Their appearance would have resembled what most people typically think of when they evoke the term “cave man.” Homo ergaster was also the first to have what we would recognize as a human voice and probably expressed themselves with rudimentary speech. With an average brain size of 700-900 cm3 (twice the size of his australopithecine ancestors), they could now contemplate more complex tools and were the first to develop the Acheulean double headed ax. In addition to tool making, this genus possessed another breakthrough technology–fire. Fossil sites show the charcoal remains usually associated with campfires. In addition to producing light and offering protection, fire offered the opportunity to cook one’s food. This produced a more easily digestible meal, further satisfying his brain’s ever increasing need for protein. Anthropologists also believe that the communal settings of a hunt and campfire, coupled with vocal capability set the stage for increased social interaction and cooperation. These traits allowed Home ergaster to survive the perils of life against larger, stronger, and swifter adversaries. Furthermore, this became a forward propelling cycle. A larger brain enabled more complex tool making and hunting strategies, which in turn required more cerebral development to communicate and teach this knowledge to others and the young. Increased mental capacity, in turn, prompted even more sophisticated planning, that required even more cerebral capacity, and so forth. Around 300,000 years ago, after countless millennia of this cycle, brain size had grown to 900 – 1100 cm3, close to that of modern humans.
Home ergaster not only fared well on the African continent but also spread to Europe and Asia, becoming known as Homo erectus (upright man). Over the next million years the eastward lineage would make it as far as China (Peking Man) and Java (Java Man). There is even evidence of populations in Europe. Currently, archeologists seem split on whether or not Homo erectus is a direct line to modern humans or a side branch that eventually died out 300,000 years ago. However, archeologists do agree that modern humans originated from the line that remained in Africa.
Scientists originally grouped