Skulls and skills varied in archaic Homo erectus

In the harsh desert badlands of the Afar triangle in north-eastern Ethiopia, the earth is slowly giving up its secrets. Bones and stone artefacts are rising to the surface, bringing with them insights into the life of Homo erectus, our ancient human relative who lived in the region more than a million years ago.

Homo erectus was possibly the most successful and longest surviving of any early human. They first popped into the fossil record some 2 million years ago and only went extinct in the last 50,000-100,000 years.

During that time, the species traipsed out of Africa, through the Caucasus, and all the way onto the Indonesian islands of southeast Asia, where their discoverer dubbed their representative specimen Java Man.

Now, scientists working at two separate sites at Gona in the Afar triangle have discovered skulls and stone tools that suggest Homo erectus was even more adaptable than previously thought.

The finds, published in Science Advances include two Homo erectus skulls at sites 6 kilometres apart, dating to 1.26 and 1.5-1.6 million years ago.

At both sites, archaeologists uncovered stone tools and artefacts close by, and in some cases encrusted with the same sediments the skulls were found in.

The surprise was that the stone tools weren’t just of a single type. Simple Oldowan artefacts as well as more sophisticated Acheulian hand axes were found.

That challenges the traditional view that different stone tools were made by different species, according to palaeoanthropologist Michael Rogers from Southern Connecticut State University, US, who specialises in stone tool analysis.

Oldowan tools – also known as Mode 1 tools – are made by smashing two rocks together to form a sharp flake.

“It’s the most basic kind of percussive technology you can imagine,” says Rogers, and has traditionally been associated with ‘handy man’ Homo habilis, a predecessor of Homo erectus.

Acheulian tools (Mode 2), on the other hand, are made by repeatedly chipping away at a rock to shape it into a hand axe, says Rogers.

The two Gona sites suggest that Homo erectus made both tool types concurrently and for several hundreds of thousands of years.

“The evidence suggests that we only have one species and yet we do have a diversity of stone tools, so that we can attribute that diversity to one species – Homo erectus,” says Rogers.

Archaeologist Mark Moore from the University of New England in Armidale, Australia, isn’t sure the distinction is so clear cut. The split between Acheulian and Oldowan is a “false dichotomy,” he says. A better approach, he suggests, would be to consider these tools as “two parts of the same technological continuum.”

Regardless of how they are classified, the mix of artefacts does indicate a flexibility in the technologies that Homo erectus employed while it was alive.

The skulls themselves are also noteworthy. One skull, which belonged to a fully-grown young adult has a brain capacity of just 600 millilitres, making it the smallest Homo erectus skull to be discovered in Africa.

“It really does look like just a miniature version [of Homo erectus]. It’s got these protruding brow ridges that are kind of classic Homo erectus, except it’s just small,” says Rogers.

The larger skull is a full 50% bigger.

The discrepancy could be because males and females were different sizes – a phenomenon biologists term ‘sexual dimorphism’.

The text-book version of early human history is that sexual dimorphism was already waning by the time Homo erectus came along.

But finds over recent years – including from the 1.85 million-year-old site of Dmanisi in Georgia – suggest that might not have been the case.

It’s likely that Homo erectus was a species of larger males and more delicately boned females. Although Rogers notes that there could be other explanations – such as variability at the population level, rather than between the sexes.

If Homo erectus was sexually dimorphic, then the reduced sexual dimorphism in modern humans and Neanderthals is more recent than previously thought, occurring sometime in the last million years.

The skull variability should not come as a surprise, according to palaeoanthropologist Philip Rightmire from Harvard University, who discovered the variable-sized skulls at Dmanisi but was not involved in the Gona work. “Biological variation is ubiquitous,” he says.

The difficulty, says Rightmire, is that the variability makes distinguishing between different species of early humans even more difficult.

“Once again, the discovery of new fossils has tended to make the overall picture of human evolution more opaque, rather than any clearer,” he says. “Reading the evidence correctly will be a challenge.”