Taung, 2.3 Million Years Ago – Scratched bones and fossil primate teeth as keys to a lost world

Raymond Dart with the "Taung child" skull, photographed in 1925. From Laitman, 1986.
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Some of us have enough trouble finding the food we want among the ordered aisles of a supermarket. Now imagine that the supermarket itself is in the middle of a vast, featureless wasteland and is constantly on the move, and you begin to appreciate the challenges faced by animals in the open ocean.

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Thriving habitats like coral reefs may present the photogenic face of the sea, but most of the world's oceans are wide expanses of emptiness. In these aquatic deserts, all life faces the same challenge: how to find enough food. Now, a couple of interesting studies have shed new light on the tactics used by predators as large as sharks and as small as bacteria.

Big fish...

At a large scale, predators like sharks and tuna rely on chemical cues to give away the location of their prey. Sharks are particularly expert trackers, but powerful though their super-senses are, they can only come into play within a certain range. Over the large distances of the open ocean, they are more like blind hunters, hoping to stumble across some telltale sign of food.

David Sims from the UK's Marine Biological Association found that many large marine predators use a search strategy called a 'Levy walk', although in this case it's more of a swim. The strategy is formally described by a mathematical equation, but in simple terms, it means that an animal makes several short moves in its search for food, interspersed with a few long ones. The longer the 'step', the more infrequent they are.

In practice, using a Levy walk means that searching a smaller area thoroughly before making a long journey to a completely new one, rather than gradually combing across the ocean. It's similar to someone looking for their keys by focusing on specific corners of a room at a time.

Sims used data from depth-sensitive electronic tags to study the behaviour of five very different marine predators as they dived and surfaced in search of food. Amazingly, all five species - basking sharks, bigeye tuna, Atlantic cod, leatherback turtles and Magellanic penguins - showed the same search strategy, one that was very close to the ideal Levy walk.

Sims believes that the predators use this strategy because it's tailored to the distribution of their prey, such as krill, which tends to be highly concentrated in specific areas and scarce over long distances. With patterns like these, a Levy walk gives a hunter a greater chance of blindly stumbling across some prey than a purely random search.

The strategy isn't universal though. Sims also tested two species, the catshark and the elephant seal, that did not use Levy walks. Nor did a young basking shark, less than a year old, which suggested to Sims that juveniles actually learn the strategy by gaining experience about how their prey is scattered throughout the ocean.

...little bacteria

Like sharks, marine bacteria also track their prey with chemical cues and depend on unreliable and unstable food sources. In this case, their meals are the leaked nutrients and waste matter produced by plankton. This organic debris is concentrated in small patches that can be a thousand times richer than the surrounding seawater, but they are prone to being swept away or diluted by currents and eddies.

So unlike the multi-celled predators, marine bacteria have another foraging challenge - they need to reach food sources quickly before they dissipate. To see how they do this, Roman Stocker from the Massachusetts Institute of Technology put an oceanic species called Pseudoalteromonas haloplanktis through a custom-built obstacle course.

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Using a technique called microfluidics, Stocker loaded the bacteria into a carefully carved series of microscopic channels and followed their pursuit after nutrients. He injected nutrients into the channels in ways that simulated two common types of oceanic food sources - a burst of nutrients caused by an exploding cell, or the long nutrient plumes that trail behind sinking organic matter - the so-called 'marine snow'.

In both these simulations, the bacteria zeroed in on the nutrients in record time, forming intense hot spots within minutes and well before their food diffused into the surrounding water. In the image on the right, the blue spots are bacteria homing in on a plume of nutrients that is strongest in the white areas and weakest in the black ones.

The key to P.haloplanktis's remarkable swimming speed is a very fast motor that drives its flagellum (its tail). Driven by the need to reach disappearing food, it can reach a top speed of 68 micrometres/s, more than twice as fast as the gut bacterium Escherichia coli, which lives a comparatively easier life constantly surrounded by nutriment. Other species of marine bacteria can even sprint at an enormous 445 micrometres/s, covering more than 200 times their body length in under a second.

Moving at such speed takes a vast amount of energy, for the flagellum's motor is an incredibly inefficient machine and becomes increasingly energetically demanding the faster it turns. Nonetheless, the benefits of reaching food before it disappears must go some way toward outweighing these significant costs.

References: Sims, D.W., Southall, E.J., Humphries, N.E., Hays, G.C., Bradshaw, C.J., Pitchford, J.W., James, A., Ahmed, M.Z., Brierley, A.S., Hindell, M.A., Morritt, D., Musyl, M.K., Righton, D., Shepard, E.L., Wearmouth, V.J., Wilson, R.P., Witt, M.J., Metcalfe, J.D. (2008). Scaling laws of marine predator search behaviour. Nature, 451(7182), 1098-1102. DOI: 10.1038/nature06518
Stocker, R., Seymour, J.R., Samadani, A., Hunt, D.E., Polz, M.F. (2008). Rapid chemotactic response enables marine bacteria to exploit ephemeral microscale nutrient patches. Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0709765105

On December 23, 1924, the Australian anatomist Raymond Dart chipped away the last bit of rock encasing the skull of a small fossil primate. The specimen had been part of a collection of fossil scraps sent to him from a limestone quarry in Taung, South Africa – not too far from where he was teaching anatomy in Johannesburg’s University of Witwatersand – but it was unlike any prehistoric primate Dart had seen before. Even though the skull was clearly from a juvenile individual, Dart was still impressed with its flat face, human-like dentition, and large brain (the convolutions of which were preserved in a fossilized cast) – characteristics which hinted at its close relationship to our species. While most paleoanthropologists at the time thought that humans had evolved in Asia, Dart believed he had found evidence that Africa had been home to the earliest humans, and he rushed a paper to Nature describing this new creature. He called it Australopithecus africanus – the “southern ape from Africa” – and this first specimen became popularly known as the “Taung child.”

Dart was sure he had found a creature which closed the gap between apes and the first humans. His A. africanus was not “a caricature of precocious hominid failure”, as he appraised the famous “Pithecanthropus” (today known as Homo erectus), but was instead “a creature well advanced beyond modern anthropoids [i.e. great apes] in just those characters, facial and cerebral, which are to be anticipated in an extinct link between man and his simian ancestor.” Yet Dart went a step beyond this. At the time it was thought that prehistoric climates in South Africa had not changed very much since the time the last dinosaurs disappeared, and the fact that the fossil was found at a site along the margin of the harsh Kalahari Desert meant that this early human had lived in a very harsh environment. It was this landscape which had made us human, Dart argued, as the evolution of humans required “a more open veldt country where competition was keener between swiftness and stealth, and where adroitness of thinking and movement played a preponderating role in the preservation of the species.” It was the crucible of our evolution.

Unfortunately for Dart, other anthropologists were reluctant about admitting A. africanus to the human family, especially when so many fascinating discoveries were being made at Dragon Bone Hill in China, but he would ultimately be vindicated. His Taung child represented an early species of human – or hominin, in today’s jargon – after all, but the ecological milieu in which it lived was quite different from what Dart had presumed.

Contrary to the well-known pirate aphorism “Dead men tell no tales”, every fossil skeleton has multiple stories to tell. Fossilized bones contain clues as to the evolution of the species they represent, the life (and, often, death) of that individual, and the kind of environment in which that organism lived. This latter class of clue is subtle, but if you know where to look it is possible to begin to reconstruct what certain places were like in the distant past, and some hints about the habitat around Taung about 2.3 million years ago can be found among the remains of fossil baboons.

As reported by paleontologists Frank L’Engle Williams and James Patterson in the latest issue of the journal PALAIOS, the microscopic damage preserved on the second molar of fossil primates provides one way to peer into the ecological history of Taung. These pits and scratches were made by different kinds of plant food as the animals chewed some of their last meals. A baboon which primarily fed on grass would have many scratches on its teeth but few pits, one which subsisted on leaves would have few pits or scratches, and one which specialized in hard foods (such as seeds and nuts) would have many pits and scratches. By looking at all these trends together – as seen on part of the second molar called the paracone – the scientists hoped to gain an overview of what kinds of plants were present in the area, and this information, in turn, would provide hints as to what kind of ecologial setting the primates lived in. To do this, Williams and Patterson made second molar casts for the baboons Parapapio antiquus (8 specimens), Papio izodi (12 specimens), and 10 indeterminate specimens from Taung, and they compared the microwear seen on these teeth with tooth damage among baboons from a similarly-aged site at Sterkfontein (also in South Africa) and the extant Chacma baboon (Papio ursinus).

A plot of the frequency of pits and scratches on the primate teeth studied. While the living baboon Papio ursinus (represented by the star) has some overlap with the "leaf browser" niche, all the fossil species do not fit neatly into the "tropic triangle" of browsers, grazers, or hard-object specialists (i.e. there are many pits on their teeth, but few scratches). Key: Star - P. ursinus; X - P. izodi; cross - Pp. antiquus; circle - Pp. jonesi; triangle - Pp. broomi; and rectangle - indeterminate. From Williams and Patterson, 2010.
Balloons_in_the_sky.jpgNineteen months ago, I wrote my first article for this blog and yesterday, I wrote my 200th. It continues to be a fantastically rewarding experience. I think that it's made me a better (or at the very least, a more careful) writer and most importantly, it's given me a way of keeping up with the awesome research that comes out on an almost daily basis. I'm not usually that good at keeping up with new hobbies and I think sticking with this one for a comparatively long time is testament to how enjoyable it is. On any typical day, there's a small pile of papers on the left side of my desk that I'm dying to write about and, if anything, there's never enough time to cover everything I want to. Hopefully, the steadily growing site traffic means that you lot are getting something out of it too. I'm enormously grateful to everyone who's read and supported the blog since its original inception at WordPress and heartily welcome any new readers who've signed up since the move to ScienceBlogs. I'm adamant about sticking to the original mission statement in these new surroundings and so far, I'm loving the insightful comments people have left behind and the added motivation to write more posts. Hope to see you all at the 300 mark. If you want to check out the last 200, they're all listed in the back-catalogue. E

When the scientists compared the data taken from the different baboon molars, they found a disjunction between living and fossil species, and even relatively clear differences between fossil genera. Much like the living Chacma baboon, the fossil species Papio izodi appeared to have a flexible feeding strategy with a wider range and pit and scratch patterns, whereas the three Parapapio species (one from Taung and two from Sterkfontein) had wear patterns which fell more closely together, as did most of the indeterminate specimens. Despite some overlap, each species apparently had slightly different diets, yet none of the fossil species fell within the classic “browser, grazer, or hard-object specialist” triangle so often used in these kinds of studies. Taken as a whole, the fossil baboons showed a variable frequency of pits on their teeth – from few to many – but there was a general paucity of scratches. What kind of diet would produce pits, but few scratches?

As recognized by Williams and Patterson, Chacma baboons regularly eat corms, roots, and tubers – foods often placed under the larger heading of “underground storage organs.” Since they need to be dug out of the ground, these foods are often covered with grit which can cause pitting on teeth, but whereas Chacma baboons often brush or wash off this extraneous matter prior to consumption, the fossil baboons may not have done the same. If this were the case, Williams and Patterson hypothesize, it could explain the high number of pits and low number of scratches on the baboon teeth from Taung and Sterkfontein. The question is what kind of environment this pattern suggests. Underground storage organs are found in both dry environments and relatively more lush environments along rivers, and while the authors favor the latter setting for Taung, there is relatively little discussion of why their study supports this interpretation.

Interestingly, the cause of death of these baboons might provide stronger hints as to what Taung was like 2.3 million years ago. Many of the fossil monkey remains were not individuals who just happened to expire there, but had been brought there by predatory birds, as was the Taung child. These primates were victims of large raptors – much like many monkeys in tropical jungles today – and the bones of many primates and medium-sized mammals found at Taung show characteristic scratches created by the feeding habits of these birds. It is another startling case of predators creating part of the fossil record through their feeding habits (much like the giant hyenas which created the Dragon Bone Hill assemblage and the “horned” crocodile which fed on Homo habilis), and as L.R. Berger and R.J. Clarke hypothesized when they announced this discovery in 1995, it probably means that Taung was once a more forested habitat, with denser cover along waterways – a suitable habitat for a large bird with a taste for primates.

What this means for the habitat at Taung is that, even though the local ecology was still becoming drier and grasslands were expanding, at about 2.3 million years ago it was an open woodland – a forest in which there were many trees but little shade. It was not the dry, scrubby habitat which can be seen around Taung today, nor was it the open savanna seen in other parts of South Africa. The climate and ecology of South Africa was not as stable as had been presumed 100 years ago.

During Dart’s time, it was popular to create heroic origin stories about the early evolution of our lineage. Dart disagreed with other paleoanthropologists over where humans originated, but parties on both sides of the argument believed that an open, harsh habitat was required to drive our evolution – had our ancestors stayed in the forest, our lineage may never have reached its full potential. Some of these stories are considered in Misia Landau’s excellent study Narratives of Human Evolution, but we would be foolish to think that, at the beginning of the 21st century, we have given up spinning such yarns. On the contrary, where Dart used the hypothesis of environmental stability to bolster his argument for human evolution, rapid environmental change is now charged with keeping the tempo of human evolution, with popular-audience programs like the recent PBS series Becoming Human suggesting that we are adapted to change itself. There is a true story of human evolution – of the lives and deaths of humans which existed over the past six million years – but we are constantly tempted to give these stories a more dramatic veneer, a gloss which flatters our egos just enough for us to believe in an inevitable “rise from the ape.”

DART, R. (1925). Australopithecus africanus: The Man-Ape of South Africa Nature, 115 (2884), 195-199 DOI: 10.1038/115195a0

WILLIAMS, F., & PATTERSON, J. (2010). RECONSTRUCTING THE PALEOECOLOGY OF TAUNG, SOUTH AFRICA FROM LOW MAGNIFICATION OF DENTAL MICROWEAR FEATURES IN FOSSIL PRIMATES PALAIOS, 25 (7), 439-448 DOI: 10.2110/palo.2009.p09-116r

Berger, L. (1995). Eagle involvement in accumulation of the Taung child fauna Journal of Human Evolution, 29 (3), 275-299 DOI: 10.1006/jhev.1995.1060

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