Australasian Science Magazine

The skull of Australopithecus sediba from Malapa in South Africa. Photo by Brett Eloff courtesy Lee Berger

By Andy Herries

Recent excavations in Kenya have revealed the first evidence that a diet of fish and crocodiles two million years ago may have aided the development of larger brains in the human lineage.

The oldest evidence of human cultural remains, in the form of stone tools, is 2.6–2.5 million years at Gona in Ethiopia. These stone tools are attributed to the Oldowan stone tool industry (named after Olduvai Gorge in Tanzania, where they were first found).

The Oldowan is a very simple stone tool industry consisting of cobbles that have had small flakes removed from them to make a type of tool known as choppers. Both the flakes themselves and the choppers were then used as tools, but what were the tools used for?

Residues left on stone tools from about 2.0–1.4 million years in Kenya suggests that the tools were used for a range of activities including processing wood, plant remains and also processing meat. It is unlikely, given the simplistic technology, that the makers of the stone tools were hunting animals with them.

The closest relatives of human tapeworms are tapeworms that affect African hyenas. This suggests that our early ancestors shared saliva with hyenas, most likely from scavenging from carcasses.

Studies of younger Oldowan stone tools and associated animal bones suggests that in many cases humans were accessing the carcasses of dead animals after carnivores had already eaten much of the meat. Humans were seemingly scavenging any remaining meat from the carcasses as well as accessing a source of protein that was not accessible to the carnivore hunters: bone marrow. It has been suggested that Oldowan choppers were used to break open the long bones of scavenged animal bones to remove this marrow, and this activity leaves distinct traces in the form of percussion marks.

While the earliest stone tools from Gona are not directly associated with human-modified animal bone, such bones are found nearby in similar-aged deposits. While only a small number occur, it is evident that humans were accessing at least a small amount of meat by 2.6–2.5 million years at the dawn of known stone tool technology.

The question is how and when meat became a significant part of the diet of early humans, and which early human species was responsible for this behaviour.

However, do these really represent the first tool technologies of early humans? Many researchers have postulated that the first Gona Oldowan tools are already relatively complex, so something must have preceded it. Such tools may have consisted of naturally flaked stone or rocks used for crushing or as anvils.

Intriguing insights into such technology has recently come to light in Ethiopia, not in the form of the tools themselves but in the form of marked animal bone. The surprise is that these bones have been dated to around 3.4 million years ago, at least 800,000 years older than the oldest stone tools. Shannon McPherron, an archaeologist at the Max Planck Institute for Evolutionary Anthropology, and co-authors suggested in their Nature paper that stone tool flaking may have been practised at this time, but that it was so infrequent and distributed about the landscape in such a way that it is “invisible” to archaeologists.

These finds may represent the first evidence for meat-eating in the diet of early humans, but when did meat become a significant part of the diet of early humans, and which early human species was responsible for this behaviour?

At 3.4 million years ago, only one early human species is known from Ethiopia, Australopithecus afarensis, an ape-like hominid that has never been associated with toolmaking. In contrast, in the period 2.4–1.7 million years ago there are a number of human species on the African landscape that may have evolved from

Au. afarensis and into the first species of our own genus, Homo. Some of these human species have been designated as Australopithecus or Paranthropus – essentially small-brained (<500 cc cranial capacity), upright-walking apes and others have been designated as Homo but there is much debate on the assignment of these fossils to a particular species or genus.

One of these hominids, Au. garhi, was found at Bouri in Ethiopia at 2.5 million years. While it is not associated with stone tools, it is associated with human modification of bone despite its small brain size (~450 cc).

Another species, Paranthropus bosei (~550 cc), is associated with a large number of stone tools and human-modified bone at 1.8 million years at Olduvai Gorge in Tanzania. Until recently this was considered the earliest evidence for humans incorporating significant quantities of meat into their diet. However, the anatomy of P. bosei has led many to suggest that this species was entirely vegetarian in nature and that the association is coincidental.

Another species found at Olduvai Gorge during this period is H. habilis. This is one of two potential species that represent the oldest evidence for Homo. The other is a slightly larger form, H. rudolfensis, and is also known from 1.9 million years in Kenya. Both these species show an increase in brain size (~700–750 cc) compared with earlier and contemporary australopithecines.

Brain size must be measured in relation to body size as larger bodies need larger brains. The overall body size and stature of H. habilis is similar or perhaps even smaller, meaning that its increased brain size is relatively greater. In comparison, H. rudolfensis has a larger body so it has a slightly larger brain than H. habilis.

A number of earlier fossils (from 2.4 million years) have been found in eastern Africa but their association is not clear other than that they are perhaps best associated with one of these forms of early Homo. These early Homo fossils may be the best candidates for the early toolmakers.

More recently even more candidates of early Homo, or ancestors to later species of Homo, have been suggested from South Africa. This includes a new species of Homo-like Australopithecus, Au. sediba, which was dated by Paul Dirks (James Cook University), Robyn Pickering (University of Melbourne) and I to around 1.9 million years ago, and a potential new species of Homo (H. gautengensis) described by Darren Curnoe (UNSW) and dated by us to between 1.8 and 1.5 million years ago. Both were dated using a combination of dating methods including palaeomagnetism (see box, p.33).

One of the more complete fossils of this species, StW 53 from Sterkfontein Cave near Johannesburg, has stone tool cut marks on it, suggesting that different species of early human may have been eating each other or perhaps even members of their own species. Evidence of cannibalism is not uncommon in human history, and has also been suggested among Neanderthals in Europe.

In contrast, Au. sediba has been suggested by its discoverer, Prof Lee Berger of the University of the Witwatersrand, to be a transitional species between earlier australopithecines and another early human ancestor, H. ergaster. Au. sediba dates to around 1.9 million years ago, soon after the last representitive of Au. africanus (Mrs Ples) was found at Sterkfontein Cave (~2.2–2.0 million years ago). The first evidence for H. ergaster occurs by 1.7 million years.

One area where Au. sediba is significant is in its rear teeth, which are smaller than other australopithecines and more like species in the genus Homo. Some researchers suggest this in itself is significant enough for Au. sediba to actually represent Homo and not Australopithecus, despite its small australopithecine-like brain.

If the Au. sediba fossils are australopithecine then it begins to cast doubt on earlier fragmentary fossils assigned to Homo in eastern Africa, as their inclusion in Homo is based in part on their small rear teeth and there is little evidence in these early fossils that their brain size had increased.

More complete skulls of Homo do not appear in eastern Africa until after 1.9 million years, around the same age as Au. sediba and only just before the occurrence of the larger-brained H. ergaster at 1.7 million years. H. ergaster has an even larger brain size than H. habilis or H. rudolfensis (~900 cc), and has a body size similar to more modern humans. Many researchers are happier with H. ergaster representing the true beginnings of the genus Homo with its combination of a more modern body, teeth and larger brain.

Overall, brain size increases more than would be expected in humans if it was merely related to changes in body size, and there is a measurable change in the brains and body size of humans in the period between 2.6 and 1.7 million years, when the first stone tools and evidence for human-modified bone occurs. Sometime during this period, small-brained Australopithecus evolved to larger-brained Homo ergaster. What happened during the period between 2.6 and 1.7 million years to account for both the changes in biology and culture? The problem for deciphering this is that it is extremely rare for both human fossils and stone tools and bones to be found in the same layers as each other during this time period.

The morphological and cultural changes that occurred could be related to climate change in Africa, with increased aridification occurring throughout this time period. Evidence also suggests that climate change may have occurred in different parts of Africa at slightly different times.

New research published by myself and colleagues in the Proceedings of the National Academy of Sciences has described significant behavioural changes that occurred. We know that early humans were beginning to incorporate some amounts of meat into their diet by at least 2.6–2.5 million years, just before we see the first increases in brain size. Excavations at the Oldowan site of FwJj20 to the north-east of Lake Turkana shows that, slightly more than 1.95 million years ago, our ancestors incorporated a far wider variety of foods in their diet than previously suspected, including animals rich in brain-growing nutrients. Incorporating these lake and river animals – including fish, turtles and crocodiles – into their diets could have played a critical role in fuelling more human-like brains in our early ancestors, as seen in Homo ergaster from 1.7 million years.

An international team of scientists led by David Braun of the University of Cape Town and Jack Harris of Rutgers University has discovered thousands of stone tools and fossilised bones, ranging from small bird bones to hippopotamus leg bones in direct association with each other. Using a variety of techniques, the team was able to determine that at least ten individual animals were butchered by early humans at this site.

Aquatic foods in particular are really important sources of long-chain polyunsaturated fatty acids and docosahexaenoic acid, which are critical to human brain growth. Finding these foods in the diets of our early ancestors suggests that they may have helped to remove nutritional constraints on brain and body size.

Gaining access to smaller animals like turtles and fish may also have allowed these small-bodied early humans to increase the protein in their diet without risking conflict with dangerous animals such as lions and hyenas. Access to such food sources may have eventually led to the evolution of the bigger brains and bodies seen in Turkana boy, a H. ergaster fossil, by 1.5 million years.

This is known as the expensive tissue hypothesis, where animals eating low-nutrient plant food tend to have more complicated digestive systems and need to devote more energy to digesting these foods. In comparison, carnivores with diets characterised by protein and fat-rich foods have more streamlined digestive tracts and need to consume less energy for digestion. These species often have bigger brains.

Humans may have had an additional benefit. With stone tools they were also partly processing this high protein meat before they tried to digest it. This may initially contributed to the reduction in tooth size that we see by 2.4 million years followed by the increase in brain size that we see from about 1.9–1.7 million years ago.

Such adaptations may also have allowed these early humans to live in a more diverse range of environments and allow them to leave Africa for the first time, as shown by the ~1.7 million-year-old human fossils found at Dmanisi in Georgia. The Dmanisi hominins carried with them an Oldowan technology that is not too dissimilar to the tools found at FwJj20.

Andy Herries is Senior Research Fellow at the University of NSW School of Medical Sciences. He helped estimate the age of the archaeological remains at FwJj20.

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Palaeomagnetism

The Earth has a magnetic field that currently runs from the North to the South Pole. This is known as a “normal” field state. However, this has not always been the case, and 780,000 years ago the Earth’s magnetic field ran from the South to the North Pole in what is known as a “reversed” field state.

Most rocks and sediments contain some form of magnetic minerals. When these minerals are deposited, they align themselves with the direction of the Earth’s magnetic field and become fossilised in the rock. The same occurs in river or lake sediments or in caves when the sediment falls out of suspension in water. In volcanic rocks the same happens when the rock cools.

As such, the direction of the Earth’s magnetic field through time can be recovered from such sedimentary sequences. The pattern and age of these 180° flips in the Earth’s field is already known from sea floor spreading zones and volcanic sequences.

The sequences of reversals that occur at archaeological sites can therefore be measured and compared against this globally known record or “Geomagnetic Polarity Timescale”, and an age assessment can be made.

The age of a volcanic layer overlying the FwJj20 site in northern Kenya was already known to be ~1.9 million years based on a radiometric age (based on the rate of decay of one radioactive isotope to another). The site lay approximately 14 metres below this, so it is older than ~1.9 million years. A magnetic reversal occurred slightly before this at 1.95 million years ago, so it was a question of locating this reversal in the sediments and comparing it with the site’s location.

As it turns out the FwJj20 site is located just below this magnetic reversal, dating it to slightly older than 1.95 million years. In contrast, the Malapa site where the Au. sediba remains were discovered was slightly younger than this reversal and is around 1.9 million years old.