Homo Erectus Was Sexually Dimorphic, New Evidence Shows

Experts explain to Haaretz how mere fragments of two skulls discovered in Ethiopia can be distinguished as male and female, and did it really use both crude and clever tools at the same time?

Send in e-mailSend in e-mail
The female cranium, dated to about 1.5-1.6 million years  ago
Apparently female Homo erectus cranium, dated to about 1.5-1.6 million years agoCredit: Michael J. Rogers, Southern Connecticut University

Homo erectus was sexually dimorphic: the males were bigger than females, scientists hypothesize, based on exactly two partial braincases discovered in Gona, Ethiopia. In addition, as these skulls were associated with both crude "Mode 1" and relatively complex stone "Mode 2" tools, they also deduce that this archaic species may have been more flexible in its tool use than had been supposed.

The crania were both discovered in Gona, the Afar region of Ethiopia, but the two erectuses lived very far apart, temporally speaking. One has been dated to 1.26 million years ago and the other to 1.6 to 1.5 million years ago, report Sileshi Semaw of the Spanish National Research Center for Human Evolution and colleagues in Science Advances.

In other words, the difference in time between them is roughly the same as the entire evolution of anatomically modern humans, which is now thought to have begun at least 300,000 years ago.

For its part, Homo erectus is believed to have evolved around 2 million years ago in Africa, and to have headed to Eurasia about 1.9 million years ago. Once there they split further into various hominin species, until eventually all went extinct. We do not know if Homo erectus is directly ancestral to us; our own evolution is a knotty conundrum. In any case, some also stayed in Africa, including in Ethiopia.

To be or not to be erectus

It has become increasingly clear that various hominin species coexisted in both Africa and Eurasia, until around 30,000 years ago. Which begs the question, given the scantiness of the remains, how confident are they in identifying these two as Homo erectus?

Reconstruction of Homo erectusCredit: frantic00 / Shutterstock.com

“The larger seems fairly straightforward: the various shape indices of the skull (including the brow ridges) are classic H. erectus,” co-author Prof. Michael Rogers of Southern Connecticut State University tells Haaretz. “The smaller cranium is more open to interpretation, but again the overall shape and projecting brow ridges look most like H. erectus, in our opinion.”

These erectuses theoretically might have coexisted with other hominins, who may have made the tools found in proximity to the crania, he acknowledges. “However, there is no fossil evidence of a different hominin species (after 50 years of searching!) from the Afar region of Ethiopia between 1.8 million years to 1 million years ago. For that reason (and because of the direct associations), we argue that H. erectus made both,” he says.

Also, since these two are not the only erectus remains ever found, they can be seen within the bigger picture, spotty as it is.

Asked how many erectus remains have been found altogether, co-author Scott Simpson, the team paleontologist, kindly explains why there is no simple answer. Most erectus remains are incomplete; many are controversial, with scientists disputing their possible identity. Also, actual skulls – crania with jaws – are very rare. Most found so far were in China and Java, Indonesia; another five were found in Dmanisi, Georgia (the location of the oldest hominin remains ever found in Eurasia). Isolated specimens were found in Turkey, Eritrea, Olduvai Gorge in Tanzania, Turkana Basin in Kenya and South Africa and there may be others.

Baboons do exhibit sexual dimorphism, as is common among primates: The male is the big one on the left.Credit: Biosphoto / Cyril Ruoso / AFP

“Depending on your taxonomy, Morocco may have some fragments, although may be too young for African erectus,” Simpson adds. “So for African materials, the Gona specimens add significantly to our understanding of the anatomy of African erectus. Also note that the [older, supposedly female one] has the smallest cranial capacity of any African erectin.”

So, let us also assume that both the skulls were erectuses.

Occam’s primordial razor

The bigger, more robust cranium is the later one, from 1.26 million years ago. The team believes that one was male, and that the earlier, relatively gracile, narrower one (1.5 to 1.6 million years ago) was female. In other words, just like gorillas, baboons, us and most primates – Homo erectus evinced sexual dimorphism: The males were on average bigger and brawnier than the females, the team postulates.

Couldn’t that have been assumed? Not necessarily. Not all primates are sexually dimorphic. “For example, gibbons and siamangs (‘lesser apes’) and some species of South American monkeys are not dimorphic,” Rogers explains. 

Siamang at the Ramat Gan Safari Park, exhibiting no sexual dimorphism; with baby on tummyCredit: Tibor Jaeger
Siamangs singing and performingCredit: YouTube

Actually, for years it was thought that erectus marked the beginning of more equal body sizes between males and females – but that’s starting to look wrong, Rogers observes.

Another theoretical possibility is that the differences between the two crania stem from the more-than-300,000 years between the two specimens. Surely during that time, Homo erectus in Ethiopia underwent some changes? Maybe the later skull was bigger because the creatures evolved?

The answer to this is big-picture. “There was likely evolutionary change over those roughly 300,000 years,” Rogers answers. “However, our evidence, in combination with other finds – for example, at Dmanisi with both small and moderately sized individuals, and at Olorgesailie [southern Kenya] with a small cranium dated to 0.95 million years ago – suggests variability throughout this time interval.”

So, dimorphism is, in their opinion, the most parsimonious answer, he explains. There are also morphological reasons to think the big cranium was male and the smaller one female: In the big ’un, the robusticity of the brow ridge suggests the individual was a male, Rogers explains. The “female” specimen was a relatively delicate little thing with a gracile brow ridge, a globular vault, an uninflated maxillary sinus and small canine root. Make of that what you will.

There’s also another possibility: that Homo erectus existed in a vast range of sizes. Just think of ex-Mr. Universe Arnold Schwarzenegger standing next to diminutive, rotund Danny DeVito, or the range of sizes available for motorcycle helmets. “There is a great deal of uncertainty with respect to the natural range of variation of our ancient ancestors,” Rogers says. “Sample sizes are quite small, and spread over long distances and great time spans.” If we find more fossils, maybe we will know more.

Arnold Schwarzenegger, left, with Danny DeVito in "Junior". Arnold is pretending to be pregnant.Credit: COLLECTION CHRISTOPHEL © Universal Pictures

For what it’s worth, it seems that two dwarf hominin species evolved in southeast Asia, and at least one is thought to have possibly descended from erectus: Homo floresiensis, whose lived on Flores Island, Indonesia. The other, Homo luzonensis, who was found in the Philippines, is quite weird, could be an erectus descendant too but it has some archaic features reminiscent of australopithecus and some modern features. Of course, Gona isn’t an island where inhabitants might undergo miniaturization.

‘Hand axes are more exciting’

In other erectus news, the two Gona specimens were both found with two levels of stone tools – Mode I and Mode II.

Mode I tools have crude modifications: a few pieces chipped off to make sharp edges.; Mode II tools are fashioned by removing flakes on both sides of the stone, resulting in more refined, “pear-shaped” hand axes, the paper explains.

The prevailing theory until now had been that Mode I hand axes (so-called Oldowan style) were invented and used by earlier, even more primitive hominins, and then along came relatively advanced, upright Homo erectus and invented Mode II tools (so-called Acheulian style) – hand axes and picks. Semaw and the team postulate that H. erectus used both technologies concurrently over hundreds of thousands of years, which could indicate that species was more behaviorally flexible and diverse than predicted.

Prof. Michael Rogers, left, and Prof. Sileshi Semaw with the female craniumCredit: Michael J. Rogers, Southern Connecticut State University
Crude " Mode 1 " Olduwan-style tools found at GonaCredit: Michael J. Rogers, Southern Connecticut State University

So possibly the development of stone tool technology may have been no more linear than our evolutionary tree, which is starting to seem more like spaghetti. It seems that dear Homo erectus used both Mode I and Mode II.

Yet again, given the paucity of data, other possibilities spring to mind. Possibly the worked stones perceived as Mode I tools were by-products of the manufacturing process of making Mode II implements, Rogers explains. Many sites with Mode II tools also have Mode I artifacts, he says, but archaeologists tend to get overwrought at the Mode II pieces and overlook the humble Mode I ones. “Hand axes are more exciting after all!” Rogers points out, compared with rock hammers.

Meanwhile, numerous archaeological sites dated between 1.7 million years to half a million years ago have only Mode I artifacts. And the new study found Mode I and Mode II with both skulls, dated 1.26 and 1.5 to 1.6 million years ago. Dmanisi erectuses seem to have only made Mode I, by the way.

The implication is that some H. erectus populations made only Mode I; some maybe made Mode II tools as well; maybe they made them rarely. For example, the site of the younger, bigger erectus skull has very few Mode II artifacts: “We could have very easily missed them, which would have led us to interpret the site as a ‘Mode I’ site,” Rogers says. “Our evidence most directly suggests that local populations of H. erectus at Gona made both kinds of tools.”

Or maybe it indicates there were some stubborn erectuses who would growl at upstarts, “Get off my cave lawn, leave my Mode I hand ax alone and take that newfangled Mode II with you.”

Somewhat more sophisticated "Mode 2" Acheulian tools found at GonaCredit: Michael J. Rogers, Connecticut State University
Top view of the female Homo erectus craniumCredit: Michael J. Rogers, Southern Connecticut University