The skull affectionately known as Mrs Ples was unearthed 50 years ago. What are the new controversies in evolutionary theory, asks Lesley Cowling
MRS PLES would be hard to find at an exhibition of human fossils, if she weren’t encased in glass and mounted on a little podium of her own.
There is not much, to the untrained eye, to distinguish her skull from the skulls of the other creatures we call “hominids” – apelike humans or humanlike apes. But the schoolchildren who look at her in awe – she’s a possible direct ancestor, they are told – are unaware she is a creature surrounded by debate and controversy, and her place in evolutionary history, like many hominid finds, is still uncertain.
It’s all based on what is not that evident to the uninitiated – the age of the skulls, determined by complicated dating processes, what their features say about the creatures they once were and how one skull differs from another. And although palaeoanthropologists are expert in the techniques of reading these signs, the theories they develop from these relics of prehistory are, necessarily, speculative.
Each new discovery can dash the hopes of a would-be Darwin or confirm another’s argument. The discovery of Mrs Ples 50 years ago was one of those. Almost a quarter of a century earlier, Raymond Dart, then a professor of anatomy at the University of the Witwatersrand, announced that a skull discovered during quarrying at a lime works in Taung, near Kimberley, in 1924, was the child of a type of hominid not discovered before. He named the juvenile Australopithecus africanus, the southern ape of Africa, and it became known informally as the Taung child.
Dart’s announcement was ignored by the scientific establishment of the time, which was based in the United Kingdom and America. Some said that the remains were probably a juvenile ape or monkey and Dart couldn’t tell the difference. But the real opposition came because the find went against theories of the time that said human evolution was unfolding in Europe and Asia, but not Africa, at the time the Taung child was supposedly playing in South Africa.
Mrs Ples arrived as proof that the Taung juvenile had kin, and would have grown into an adult hominid, just as Dart had theorised. Her skull (now believed to be a male skull) showed similar features to those that had led Dart to classify the Taung child as a new type of hominid and name it. It also showed that hominids were evolving in southern Africa about three million years ago.
Now the custodians of Mrs Ples, Dr Francis Thackeray and his colleagues at the Transvaal Museum, are about to publish research that will once again shake-up many accepted notions about evolution, notions in the palaeontological world, that is.
For those of us who have never evolved beyond vague Darwinian ideas on the origin of our species, the process seems to be straightforward simple lineage – a slow change over millions of years from one form to another, starting out as upright apes and slowly developing the features that make us human – in particular, a big brain.
And, boy, do we need it when following the ins and outs of discoveries, names, skull details and ages that, in the past 30 years, have led scientists to believe that there was not a single slow lineage, but many branches of the hominid family tree.
One of the crucial discoveries, according to a book called African Exodus by Chris Stringer and Robin McKie, was the unearthing of a man from the river Kibish in Ethiopia, who had features that were more like ours than the Neanderthals of Europe, thought at the time to be our direct ancestors. But Kibish man was about 130 000 years old, much older than the Neanderthals, and evolution doesn’t work backwards. So what had happened?
The palaeontological world did a rethink and decided that Neanderthals were not our ancestors after all, but our cousins. That is, they evolved from the same ancestor, but went off on a different evolutionary path, developing in a different way to the creatures who would eventually become modern humans. Eventually, they were replaced by the forerunners of modern humans. And if Neanderthals could be cousins, so then could other creatures unearthed, named and classified.
Much debate has subsequently been generated about whether this or that creature was an ancestor or a cousin, going down the route of no-return, evolutionarily speaking.
All of this is based, not only on dating techniques, but on the measurement of skulls and fossil skeletons. And if those relics differ from each other in significant ways, they are considered to be separate species.
The way modern palaeontologists go about it is to take a host of measurements, then use a set of complex calculations to produce a number that reflects how different the objects are from each other – the sum of how much they vary.
Thackeray and his colleagues at the Transvaal Museum began measuring creatures from the same species, from insects to mice, to modern humans and fossils, all from the same time period. The aim was simple: they wanted to find out how much variation there was, generally, between individuals of the same species. The results, says Thackeray, were intriguing. “We found that there was a very similar range of variation in all the creatures we looked at.”
Individual creatures from the same species never vary more than a certain amount from each other. That amount of variation appears to be the same for all the species measured by the Transvaal Museum staff.
In simple terms, nature allows for a certain amount of difference between individuals in one species. The latitude for variation is quite large and each species has the same amount of variability. The implications for palaeontology, Thackeray explains, is that many hominid examples that have been classified as separate species fit into the same species if you allow for the usual variation. So many of our cousins are actually ancestors, and it looks like we may have single lineage after all, with a very diverse set of ancestors.
But evolutionary theorists caution against placing too much emphasis on measurement variations. They point out that the modern definition of what makes a species is not physical differences between creatures, but whether or not they can breed successfully. And whether that happened cannot be read from the hominid fossil record.
Even if you’re a fan of the variability theory, the acid test is the Neanderthals. Thackeray is going to France next month to measure some Neanderthal skulls and compare those measurements against the measurements of hominids living at the same time.
If the amount of variability is greater than the standard proportion found by the Transvaal Museum researchers, then they remain our cousins. If not, we may have to consider welcoming these much-maligned beasts back into the tribe. And we may be closer to determining whether Mrs Ples can be called a direct ancestor.