Looking at the many different species that inhabit the world today it is apparent that humans are most closely related to chimpanzees and bonobos. Data from the fossil record and from genetics show that humans and chimpanzees and bonobos had a common ancestor about 6-7 million years ago. In looking at the evolution of these two groups, scientists use the term hominin to refer to the lineage that includes modern humans and all of the extinct animals which are more closely related to modern humans more than any other living group. The term panin is used to refer to chimpanzees and bonobos (chimps/bonobos) and all of the extinct animals which are more closely related to chimps/bonobos than any other living group.
One of the earliest anatomical features which evolved to distinguish hominins from panins was bipedalism—walking upright on two legs. Members of the genus Homo, were not, however, the first to exhibit bipedalism. The evolutionary transition into the earliest members of the genus Homo, therefore, involved anatomical and behavioral characteristics in addition to bipedalism. In describing the differences between the earlier bipedal australopithecines and the early humans (genus Homo), Donald Johanson and Blake Edgar, in their book From Lucy to Language, write:
“Australopithecines are characterized as having relatively small brains, large cheek teeth, a postcranial skeleton with some apelike features, and an absence of culture. In contrast, species of the genus Homo have relatively and absolutely large brains, a more modern postcranial skeleton, a significant reduction in tooth and jaw size, and, most important, culture.”
The evolutionary transition from earlier hominins into the genus Homo is rather fuzzy with researchers linking this to rather diverse biological and behavioral attributes. The idea of culture, particularly evidenced by the making of stone tools, is one of the important behavioral traits. However, current research shows that some panins as well as pre-Homo hominins also make and use tools. So toolmaking, per se, may not be the definitive characteristic that marks the transition into Homo.
There is some controversy and debate among paleoanthropologists over the two earliest hominins who have been assigned to the genus Homo: Homo habilis and Homo rudolfensis. This debate includes the question as to whether or not these two species should be classified as Homo or not, as well as the question as to whether or not they should be considered ancestral to modern humans or as evolutionary dead ends. In their taxonomy of hominin species, Bernard Wood and Amy Bauernfeind, in their chapter in The Oxford Handbook of Language Evolution, sort hominins into six grades in which Homo habilis and Homo rudolfensis are classified as Transitional Hominins.
The first emergence of Homo about 2.5 million years ago correlated with a period of worldwide climatic cooling. At this time the Arctic and Antarctic icecaps may have become permanent. Some researchers suggest that at this time the African rainforests shrank, and the open savannahs expanded. The Turnover Pulse Hypothesis suggests that as the climate changed, hominins and other animals had to adapt to increasingly arid and open conditions. In his chapter on the fossil evidence for human evolution in The Oxford Companion to Archaeology, Leslie Aiello writes:
“Early Homo would have achieved this through the evolution of tool use, a larger brain, and an arguably more complicated social structure.”
In his book The Neanderthal’s Necklace: In Search of the First Thinkers, Juan Luis Arsuaga reports:
“For the first time, meat and animal fat were an important part of the hominid diet.”
Of the increased brain size at this time, Juan Luis Arsuaga writes:
“This increased brain size may have had to do with a new way of life, now based on more dispersed and less predictable resources than in the tropical forest. This was true for the search for plant life and even more so for the hunt for meat.”
Increased brain size may have allowed these early humans to make improved mental maps of fairly large gathering areas, to interpret animal tracks, to understand the changing seasons and to plan for the future. Juan Luis Arsuaga also writes:
“It is also very likely that their social groups expanded and became more closely integrated and cooperative, their larger brains allowing for social and behavioral patterns that distinguished them from all other animals.”
While the Turnover Pulse Hypothesis is interesting, recent evidence from stable carbon isotope analysis suggests that the climate in Africa did not change substantially throughout the period.
In comparing Homo habilis and Homo rudolfensis, Alan Mann in his chapter on the genus homo in The Oxford Handbook of Language Evolution. writes:
“H. habilis had narrower premolars and molars housed in a somewhat smaller face in comparison to the earlier australopithecines, but its brain size is only marginally larger than some australopithecine species. H. rudolfensis, in contrast, possessed a larger brain but also a larger, australopithecine-sized dentition and face. Both are troublesome to place taxonomically and suggest it may be difficult to precisely recognize the earliest member of Homo.”
In their book From Lucy to Language, Donald Johanson and Blake Edgar write:
“The evolution of Homo was not a simple linear story from habilis to erectus to sapiens, and we must now decide whether habilis or rudolfensis makes a more likely ancestor for the rest of our lineage.”
It is also possible that neither is the likely ancestor. As more ancient fossils are found, our understanding of human evolution changes. Donald Johanson and Blake Edgar write:
“For the moment, the evolutionary roots of Homo are still poorly understood, but they will ultimately be found in pre-2 million-year-old deposits.”
More Human Origins
Human Origins: Homo rudolfensis
Human Origins: Homo habilis
Human Origins: Humans as naked apes
Human Origins: Sex
Human Origins: Domesticating Fire
Human Origins: The Large Brain
Human Origins: The Mind
Human Origins: Bipedalism