Piecing together human evolution is a frustrating job. The simple family tree we once pictured is now looking more like a messy vine, and the fossil record is patchy, making it hard to establish how possible ancestral species related to each other. When we do have fossils, their ages are often unclear. A new study not only improves our capacity to identify fossils' dates, but explains some major gaps: in southern Africa our specimens are restricted to dry eras.
There has been much debate as to whether eastern or southern Africa deserves the title of the true cradle of humanity. Dr Benjamin Schoville of the University of Queensland explained to IFLScience the two have very different geologies.
In eastern Africa, Schoville described the ground as “like a layer cake” with volcanic eruptions giving us accurate dates for anything preserved beneath them. Southern African caves, on the other hand, have provided a rich source of Australopithecus, Paranthropus, and Homo remains over the last 3.2 million years. Schoville explained, however, that these are mostly not caves as we think of them, and don't indicate our ancestors were cave dwellers for much of this time. Instead, they are “more like sinkholes,” Schoville said, with human bones washed in or dropped there by birds.
Unsurprisingly, these remains are much harder to date accurately. Now, however, Schoville has reported in Nature that these sinkholes are climate dependent.
“We sampled and dated calcium carbonate flowstone rocks that form in caves, and discovered that the caves only accumulated fossils when the environment was dry,” Schoville said in a statement. “We know this because the caves were closed off when it was wetter. During these times the flowstones, such as stalagmites and stalactites, were able to grow and do not include fossils.”
Major caves were closed by vegetation and flowstones for periods of hundreds of thousands of years between 3.2 and 1.3 million years ago. “Because the accumulation of fossils was constrained to certain periods, the evolution of these early hominins looks like it happened in rapid bursts, but it may have actually been a much more gradual process,” Schoville said.
Unfortunately, Schoville explained to IFLScience, we can't do much to fill these gaps, with no likely places to look for wet-era fossils.
On the positive side, however, the flowstones can be used to date the fossils deposited between them more precisely. This, Schoville said, will “put the southern African record on a par with East Africa,” and may eventually help us understand the relationships between the species we identify.
More broadly, Schoville thinks other interrupted fossil records may have encouraged the theory of “punctuated equilibrium” where evolution is thought to occur in short bursts, separated by periods of near-stasis.