Beginning around 2.5 million years ago or a bit earlier, there was a major forking in the evolutionary path of hominins. The australopithecines diverged into at least two very different evolutionary directions. One led to the robust australopithecines and a genetic dead-end by about 1.4 million years ago. The other led to the first humans. It is likely that these diverging evolutionary paths were the result of exploiting different environmental opportunities. Coinciding with this hominin divergence was a shift in the global climate to cooler conditions. In East and South Africa, where most of the early hominins apparently lived, dry grasslands expanded at the expense of woodlands and forests. It has been suggested that the adaptive radiation that led to humans and robust australopithecines is connected with this change in the environment.
click this icon in order to see the following video Search for the First Human--Donald Johanson talks about the complex
evolutionary picture of early hominins that is emerging from the fossil record.
This link takes you to a video at an external website. To return here, you must
click the "back" button on your browser program. (length = 2 mins, 10 secs)
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Note: the Orrorin referred to in the video is Orrorin tugenensis, a possible
ancestor of the australopithecines that lived about 6 million years ago.
NOTE: This is not the only possible model of early hominin evolutionary links that has been suggested in recent years. Some paleoanthropologists have proposed that neither africanus nor afarensis were ancestral to the robust australopithecines. In other words, the division between robust and gracile forms occurred earlier, perhaps at the time of anamensis or before. It is also possible that humans descended from anamensis through a still unknown intermediate gracile species instead of afarensis. On-going research will very likely sort out the relationships between the various hominin species in the near future.
NEWS: John Novembre et.al. reported in the October 1, 2007 issue of Nature Genetics that human saliva has significantly more of the enzyme amylase compared to chimpanzees. Amylase breaks down starches into glucose which can be readily used by the cells of the body. With more amylase, humans get more useable calories from starchy vegetable foods such as tubers, corms, and bulbs. The authors suggest that this would have been a distinct advantage for early humans because these foods are readily available. They believe that natural selection favored additional copies of the gene responsible for amylase production (AMY1) in our early hominin ancestors but not in apes.
http://anthro.palomar.edu/hominid/australo_2.htm