In order to become a fossil, an animal has to die in a very special, usually very bad way. Some sort of way that preserves its body, such as falling into a tar pit, for example. Since this doesn’t happen often, fossil evidence can be hard to come by. In absence of this sort of evidence, scientists have used estimates of genetic rates of change to estimate the timing of the point when evolution split humans and chimps. Estimates diverge quite a bit, but are mostly centered on five to seven million years ago.
With the recent discovery of fossils sandwiched in volcanic ash in Ethiopia, however, scientists have actual fossil evidence to revise this estimate. It would appear that this ancestor lived about 8 million years ago, pushing back the split by about two million years, and pinning this species to this region of Africa, rather than Eurasia, as previously thought. Of course, this is still a rough estimate. The ball may have gotten started on changes as far back as 13 million years ago, with genetic changes manifesting between populations.
The new timeline appears still to be consistent with the proposal that major geological changes forced adaptations that drove the split. Specifically, the Himalayas. Because there is little impetus to change when a species is successful (such as modern-day humans), there isn’t much change beyond genetic drift, according to punctuated equilibrium. In fact, if the species is successful, there is a strong reason for individuals to stay close to the mean, rather than to change. In the case of Africa 20 million years ago, this meant that our ancestors were well adapted to lush jungles and treetops.
When the Indian subcontinent started pushing its way into Asia, though, it began pushing up the Himalayan mountain range. This enormous mountain range locked up a tremendous amount of moisture from the global currents, and probably contributed greatly to the drying out of Africa into a savannah (and possibly to the subsequent ice ages, as well).
The enormous pressure to adapt to a new environment like a savannah probably drove enormous species changes, and is probably a big part of what drove the split between humans and chimps. Adapting to a savannah landscape may have driven changes such as walking upright, which may then have freed up some of the chest muscles for fine motor coordination associated with things like speech, or regulated breathing for distance running, perhaps.
As some skeptics point out, more fossils will be needed to confirm the conclusion drawn by this latest study. For example, it is conceivable (though unlikely) that this is a tangential branch of Hominidae that never actually evolved into humans, and instead died out. For the time being, though, it’s some of the strongest physical evidence on record — and a fascinating look at the earliest roots of the human family tree.