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Meeting the Missing Link

Image of fossil skull

The cranium of the juvenile skeleton. Photo: Brett Eloff courtesy Wits University

By Paul Dirks

Paul Dirks gives a first-hand account of the expedition that found a new species of hominid linking humans and apes.

As a structural geologist I never expected to become so closely involved in such an important fossil find as Australopithecus sediba, which may be the transitional species between ape-man and the genus Homo from which we evolved (see Linking Man and Ape, p.15).

Between 2002 and 2009 I was Head of the School of Geosciences in the University of the Witwatersrand, Johannesburg, and would regularly visit the fossil sites. Then, in early 2008, I decided to start a new project with my Wits University colleague, Professor Lee Berger, aimed at understanding the geological and geomorphological setting of the fossil-bearing cave systems.

I had been toying with the idea that if fossils occur in deposits along faults and rifts, which formed as a result of uplift of the high veld in response to processes deep inside the Earth, then human evolution itself may be somehow linked to those tectonic processes as well. If we know where the caves are, and we can predict how the landscape developed, then maybe we can predict where the caves with fossils are.

We intended to map a large section of the Cradle of Humankind World Heritage Site, and were hoping that this would give us some fundamental insights into the story of human evolution and help us understand the relationships between the caves and the landscape better.

Using air photos, satellite images, and Geographic Information Systems and basic geological techniques in the field, I would map as Lee would spend time tracking down new caves and cave deposits. Over the first half of 2008 our database of cave deposits grew from 80 or so to 500–600. There were caves everywhere, many with fossils.

At the end of July, Lee had spotted an interesting line of caves on GoogleEarth that he wanted to check out, so on 1 August 2008 we set out – me to map some of the new cave sites and Lee to check out the new caves.

That afternoon Lee told me he had found a new site with lots of macrofossils in outcrop. He reckoned it looked really promising and there were lots of other cave sites nearby.

So together we walked back to the line of caves to have another look. Sure enough there were lots of caves, most excavated by lime workers, but it was getting late by the time we neared the Malapa site and we decided to turn back without looking further.

Two weeks later Lee returned to the site with his 9-year-old son Matthew and Dr Job Kibii, and it is then that he first spotted a block with hominid fossil remains clearly visible.

A few days later we all went out again. There were five or six of us, and I began mapping the geology of the site while the others rummaged around the loose blocks of rocks strewn around by the miners. Many of these blocks contained spectacular fossil bones, although to appreciate their value requires a special eye.

Within a few hours Lee had picked up piece after piece of hominid bone. It is rare to find a little piece of any hominid at the very best of times, but it is unheard of to just walk around and pick up hominid bones and teeth everywhere. At the end of that memorable day we had collected perhaps 15–20 blocks with hominid material in it.

Lee had picked up several teeth in the dirt in the bottom of the pit, and identified more material in the pit sidewall. It was fast becoming clear that we had found something very special indeed, and that there were not just one but several individual hominids.

The first block Lee had picked up with his son produced a fantastic lower mandible (or jaw) of a juvenile hominid. From there it did not take long to find other pieces, including a beautiful skull, arms, legs, vertebrae, elements of the pelvis, ankle bones etc.

It soon became clear that there were at least two individuals, and possibly more. The bone was beautifully preserved right down to the plaque on the teeth of the juvenile skull that was recovered.

The hominid fossils were a juvenile male and an adult female. By putting the rocks back together we were able to establish that they were lying close together in a coarse-grained sandstone.

Were they related? Did they die together? What had happened? Most importantly we needed to work out the age of the fossils.

We invited experts with dating experience to help us. First we looked at the other animals that occurred with the hominids, and these were amazing in their own right. There were sabre-tooth cats as well as a wild dog, a hyena, several mongoose, a horse and various antelopes, and a multitude of smaller animals. Some appeared entirely new to the scientific world and are yet to be described.

We now asked Jan Kramers, a geochemist from Bern University, and Robyn Pickering from the University of Melbourne to help us with dating a flowstone that occurred immediately below the sediba fossils. Using slightly different isotope techniques they managed to get a blind duplicate date of the flowstone sample.

The results from both labs were nearly identical and indicated that the flowstone was a little over two million years old. We now invited Andy Herries from the University of NSW to help us further constrain the age of the fossil-bearing rocks. In that way we managed to establish that the fossils were slightly younger than 1.95 million years. Armed with an age, we could now start placing the fossils into our family tree.

In the meantime Lee Berger and the palaeoanthropology team worked hard to describe and identify the hominid fossils. Based on the metrics obtained from various morphological features preserved in the fossil skeletons, they worked out that this was a new species – and not just any new species but a transitional form, maybe the best yet found, between Australopithecines and early members of the genus Homo.

The implication was huge because the sediba fossils potentially redefine the evolutionary tree at the base of the genus Homo, and they could perhaps replace other candidates such as Homo habilis as our most distant direct ancestor.

The characteristics that set aside sediba include a small brain (420–450 cm3), and long arms like an orang-utan, which make the species like members of the genus Australopithecus. But in contrast to other Australopithecines, sediba has a more evolved dentition (essentially smaller teeth) and short powerful hands like a human, and very importantly the pelvis and hip are advanced, similar to modern humans and approaching the hip structure of Homo erectus, indicating that sediba was able to walk upright in a striding manner.

Their legs appear long and their ankles seem to be intermediate between ours and earlier hominids. Because of these transitional traits Lee Berger speculates that sediba is a convincing candidate to be the immediate ancestor to the genus Homo.

The cave where the fossils were found would have been deep and hard to access when sediba roamed the area. I managed to determine that much of the overburden had been removed by erosion in the past two million years, and that the fossil deposits are probably erosion remnants that were once deposited deep inside the cave.

To explain the fossil assemblage and the well-preserved state of the fossils, we speculate that at the time of their death, the area in which sediba lived experienced a severe drought. Water became critical and could only be found deep inside narrow, steep and often dangerous cave systems were scavengers would not normally go or take their prey.

The animals and hominids may have smelled the water, ventured in too deep, fallen down hidden shafts in the pitch dark or got lost and died.

Carnivores may have also been attracted to the cave. Maybe they also smelled water, or maybe they smelled the dead animals trapped in the cave. They entered and also became trapped and died.

And then a big storm came – as is common today on the high veld of South Africa – and a torrent of water washed the various animals that died together deeper into the cave.

The project Lee Berger and I started was multidisciplinary right from the start, which has meant that we have learnt a great deal from a great many people in a relatively short period of time. We now know that we have an important new species and that the landscape in which sediba lived was dynamic and probably quite different from what the Cradle looks like today.

It has been a very exciting past 2 years, and we have only scraped the surface. There are probably many more sites with good fossils waiting to be discovered.

Professor Paul Dirks is Head of the School of Earth and Environmental Sciences at James Cook University, and former Head of the School of GeoSciences at Wits University in South Africa.