A million years ago, Dandero was a very different place. Instead of the dry, dusty badlands that characterize the area, there were flowing rivers, lakes and lush forests. Animals thrived in the much wetter climate. The proof is in the more than 500 metres of sedimentary rock stacked one on top of another, like a layer cake, for hundreds of square kilometres around. Embedded in the rock are the fossilized remains of hundreds of land mammals: elephants, hippos, antelope, pigs, horses; water animals like fish, crocodiles and turtles; and plant and invertebrate fossils like land snails. Even pollen remains are part of this unusually rich fossil record.
“The density of artefacts, the preservation of the animal bones, and their presence in a context that can be dated – precisely dated – is unusual,” says Dr. Michael Chazan, an archeologist at the University of Toronto. He’s a member of the Eritrean Research Project team, an international collaboration among Eritrean, Canadian, American, Dutch and French scientists formed to study the relics found at the site. “It has a lot of potential to tell you about the behaviour of early humans, Homo erectus in particular, and the environment in which they lived, the ecology.”
Bob Walter, director of the Eritrean Research Project, calls the whole area ‘Pleistocene Park’ because “we have such a long record of Pleistocene sediment in this area.” He’s spent the last three winters in Eritrea studying this unexplored terrain.
The Pleistocene refers to a time period between 1.7 million and 10,000 years ago. And a rough calculation of how much time is represented in the sediments of Dandero is on the order of a million years.
“At the bottom of that section, it might be a million and a half years old, and at the top, it might be 500,000 years old,” explains Walter. “So, within that entire 500-metre sequence, we might be able to see the right kinds of fossils and artefacts and other things that tells the story of evolutionary progression.”
So what do the sediments tell us?
“What we see are the kinds of sediments that are deposited in shallow, freshwater lakes. We also see river and delta sediment. Just on the basis of geology, you can get a pretty good picture that here was an area that was obviously temperate enough to support permanent lakes and flowing rivers. And when you add to that geological story, the fossils that we’re finding, then you can start to piece together what the surrounding countryside must have been like – primarily a woodland environment very close to the water, and then grading out into a brush, or savanna grassland.”
While no hominid fossils have yet been found, a lot can be said about the people who must have hunted the animals a million years ago – especially since thousands of stone tools were also left behind.
Campsites of our early ancestors
Finding fossils and stone tools in sedimentary rock layers is like finding the campsites of the people, explains Chazan.
“You’d start to look at how different artefacts are distributed. You’d look at how – in terms of sharing meat – different bones are divided across the site… and you’d try to understand how a dead carcass entered into a campsite and how it was processed.”
It becomes highly interpretive at this point, says Chazan, but the division of particular parts of an animal can tell us something about how the society was ordered.
The sedimentary rock is also witness and recording secretary to multiple climate changes, from warm, temperate periods, like the one we’re in now, to extreme ice ages.
“We see periods of these lush, wet conditions – lake and river environments – and then we see areas of sediment that are indicative of climate drying, thinly laminated beds of gypsum and salt called evaporate deposits [when the climate was much colder],” says Walter. During the Pleistocene, the world went through extreme weather cycles, enduring numerous ice ages once every 100,000 years. The animals and toolmakers whose remains are left behind in Eritrea were undoubtedly affected by global climate change. And these climate changes may also yield clues into key transitions in human evolution.
Our earliest ancestor, a bipedal primate, branched from our common ancestor with the great apes about six to four million years ago. Scientists believe that climate was key to this first evolutionary transition. At about the same time, shifting tectonic plates created a range of volcanoes north–south between East and Central Africa. This mountain range created what’s called a ‘rainshadow’ in the elevated countries to the east (Tanzania, Kenya and Ethiopia): Moist air from the Congo Basin hits the mountains, then dumps the moisture in the form of rain. But, once the air passes east over the mountain range, the lack of moisture produces a drier, hotter climate in the eastern countries.
“So you’d have a kind of chimp-like animal living all over central Africa. But, the population, isolated in East Africa in the drier conditions, started changing and eventually became humans – that would be a simplistic way of looking at it,” says Mario Gagnon, a paleoecologist at the University of Toronto and member of the Eritrean Research Project team.
A drier climate would lead to more open vegetation, so forested habitats would thin out, leaving only patches of trees isolated from each other. At this point in human evolution, trees would still provide safety – from other predators – and food. So, instead of leaping from tree to tree closely set together, our earliest ancestors evolved to walk from tree to isolated tree. “The idea is that walking on two legs would give a special advantage in that condition,” explains Gagnon.
Bipedalism would then free up two hands, which would enable our early ancestors to carry things. It would also allow for a more efficient method of moving across long distances. While walking on four legs might be faster in small bursts, two legs demand less energy when walking long distances, says Gagnon.
The second key transition occurred at about 2.5 million years ago with the appearance of Homo erectus, a member of our own genus. These early ancestors began crafting stone tools rather than picking up rocks with convenient shapes. Utilizing this new skill, they started eating meat. Then, somewhere between two and one million years ago, came the dramatic growth of the brain and our ancestor’s first migration out of Africa. At about 100,000 years ago, emerged what are called anatomically modern humans – ancestors who resembled us. This new group made a second migration out of Africa.
An exploration of climate changes in Pleistocene Park may yield clues into the latter evolutionary changes – how and why we developed bigger brains and left Africa, and how we came to resemble the Homo sapiens we’ve become.
“Eritrea is going to become, I think, a major player in terms of human evolutionary studies, particularly for this period between 1.5 million years and 100,000 years ago,” says Walter. “And that’s interesting from a human evolution standpoint because that documents the entire evolution of Homo erectus up to the transition to anatomically modern humans.”
“We’re just scratching the surface. We’re really just beginning this whole process, there’s lots of territory to cover.”