n today’s post, we are treated to the third part of a series by James Kidder on the human fossil record. So far in this series, Kidder has addressed some misconceptions about the fossil record- namely, the idea that evolutionary lineages are direct and linear. Rather, the author explains, new species arise from common ancestors in a sort of branched pattern. The shortage of “transitional species”, therefore, does not strengthen the creationist debate against evolution as many have come to believe. We have also learned about the features which set Homo sapiens apart from other primates. These characteristics, particularly bipedalism, have arisen gradually over time. Their progression can be observed in the fossilized skulls of our ancestors.
In a continuation of these thoughts, Kidder takes us back in time nearly one hundred years to the discovery of the first piece of evidence which confirmed Darwin’s idea that Homo sapiens originated in Africa. A primate skull with indications of bipedalism was found in a region of Africa where higher primates were known to be absent. It was named Australopithecines africanus by its discoverer, anatomist Raymond Dart. Dart’s monumental discovery, made by in the early 1920’s, initiated a cascade of other discoveries and identifications. Ten other species of australopithences have since been discovered in Africa. The radiation of these ancient species was extensive and lasted from about 4.0 to 1.5 million years ago. In the next part of this series, Kidder will discuss the radiation of these australopithences fossils, including the well-known Lucy of Ethiopa.
The Discovery of Australopithecus
The Taung Child
In the early 1920s, a young anatomist named Raymond Dart took a job at the University of Witwatersrand in Johannesburg, South Africa. Keenly interested in comparative primate anatomy, Dart had been advised to go to the Wit by the famed anatomist Sir Grafton Eliot Smith and, upon arrival, began work on the ancestry of South African primates.
North of Johannesburg lay the Buxton Limeworks where fossils were being blasted out by the thousands, and Dart, through a colleague at “the Wit,” informed the workers there that if they encountered anything interesting, he would be more than willing to receive it. One worker had been collecting fossils for several years and had two large crates of them. When these crates arrived in Dart’s laboratory, he examined them to see if they had any primate fossils. The first did not but the second revealed a beautiful specimen of a child’s partial skull and fossilized brain cast in it. Both were embedded in a limestone concretion, which took Dart 73 days to pry apart. Once he did, he discovered, based on the eruption pattern of the teeth, that he had the skull of a two to three year old child (see Figure 1 at left. He also noticed several odd things. While it was clearly a primate skull, the teeth, somehow, didn’t look right. As I mentioned in the last post, in all non-human primates, the canine (eye tooth) extends beyond the tooth row, even in infants. The canines of this skull did not. (See Figure 1 at left and for a detailed comparsion, see here and here.) He also noticed that, for a primate skull of two to three years of age, the brain was simply too large, being roughly twice the size of a comparable baboon skull. Further, he knew that this far south, there were no higher apes and, therefore, it was clearly not a chimpanzee skull.
The most important thing that caught his attention, however, was that, while not complete, the base of the skull revealed that the hole for the spinal column (foramen magnum, see the last post for explanation) was positioned at the bottom of the skull rather than at the back of the skull as in baboons. Dart could only conclude that, whatever this creature was, it walked upright and he took the unusual step of calling this new findAustralopithecus africanus or “Southern Ape Man from Africa.” It was the first confirmation of the prediction of Charles Darwin that the original ancestors of the human line would be found in Africa.
Dart met with considerable skepticism when he described his find in the journal Nature (Dart 1925). Not until the venerable palaeontologist Robert Broom backed his claims, did Dart’s phenomenal discovery begin to percolate through the field. Between 1924 and 1949, Broom and Dart made many discoveries of australopithecines in four other caves in South Africa. These solidified the hypothesis that these finds were not aberrant apes or deformed fossils, as some thought, but were, in fact, in a line that diverged from apes and led, eventually, to humans.
The Definition of a Species in Living Populations
Before examining the appearance and proliferation of the australopithecines, which began around 4.0 million years ago, a short assessment of the nature of palaeospecies is necessary. As the marriage of evolutionary biology and mathematics grew to yield the fruitful field of population genetics, a problem still remained: how to scientifically define a species. Even when working with known populations, the problem of where to draw the line between two groups remained.
In 1942, evolutionary biologist Ernst Mayr, working with bird collections in the American Museum of Natural History, developed the “Biological Species Concept” which was as follows: “species are groups of interbreeding natural populations that are reproductively isolated from other such groups.” (Mayr 1942) This definition revolutionized the study of biological species and populations by providing a mechanism for the formation of species and showing the importance of natural selection and its effect on the individual organism. This quickly became the de facto definition of species and helped to usher in the “new synthesis” of evolutionary biology in the 1940s and 1950s, a definition that could work for researchers no matter what they were studying…as long as it wasn’t fossils.
The Definition of a Species in Extinct Populations
Because fossils give us no information about breeding practices, population size or morphological diversity, they add yet another dimension to the species problem. The ultimate question a palaeontologist faces when holding two fossils is, “how different are these two individuals and could they conceivably have been part of the same breeding population?”
There are typically two different concepts of species definitions as they apply to the fossil record: the phenetic concept, which uses all of the observable characteristics a fossil has and relates them to other like fossils, and the phylogenetic concept, which focuses on traits one fossil has that are different from like fossils around it. As Wood and Longeran (Wood and Lonergan 2008) note:
In practice most researchers involved in hominin taxonomy use one or other version of the PySC [phylogenetic species concept]. They search for the smallest cluster of individual organisms that is ‘diagnosable’ on the basis of the preserved morphology. Because in the hominin fossil record most preserved morphology is craniodental [head and teeth], diagnoses of early hominin taxa inevitably focus on craniodental morphology.
Remember that a phylogeny is the evolutionary history of a species. In most cases, this is very hard to discern. As a result, the adaptive radiation of the australopithecines, in some ways, is a classic example of collateral ancestry that we discussed earlier (see this post) in the sense that, while we may not know who was related to whom, all of these species show transitional characteristics. As a result, most palaeontologists have treated the radiation of australopithecines as reflecting many different species. Whichever species concept one uses, the radiation, as evinced from the fossil record, was extensive, beginning around 4.0 million years ago and lasting to possibly as late as 1.2 million years ago (see Figure 2 below, adapted from Science Magazine).
The discovery by Dart of Australopithecus africanus served as a springboard for later excavators and its characteristics a template from which to compare other finds. Since Dart’s fateful discovery 97 years ago, ten more species of Australopithecus have been discovered, ranging from very gracile and petite forms to very large and robust forms. Figure 2 shows the current presumed australopithecine taxonomy. The red bars show the inferred time span of each species. Missing is the newly discovered Australopithecus sediba, whose placement is not currently known but is thought by the discoverers (Berger et al. 2010) to be descended from A. africanus. In the next post, I will address the radiation of the earliest australopithecines, including the famous “Lucy.”
For reference, Figure 3 below shows the Australopithicus specimens in time-perspective to other hominid species that have discovered so far.
Figure 3 Australopithecus in Perspective (from Science Magazine)