Monthly Archives: April 2010

Australopithecus sediba: an evolutionary mosaic

The skull of MH1, a juvenile member of the species Australopithecus sediba.

Two beautifully preserved partial skeletons of a new species of human are described in the current issue of Science magazine. The new species has been given the taxonomic name Australopithecus sediba. The remains were discovered in Malapa, South Africa, located a mere 15km from the famous Sterkfontein caves. The site preservation is incredible, especially considering its great antiquity. The specimens themselves are relatively free from distortion and show few signs of taphonomic modification.

The fossils are around 2 million years old based on a combination of radiometric and palaeomagnetic dating, as well as the associated animal remains found at the site. The skeletal remains are those of an adult female and a boy of between 9 and 13 years. A. sediba would have stood at about 1.3 metres tall and had relatively long arms like those seen in other australopithecines.

Palaeoanthropologists are split on whether these fossils are members of our genus, Homo, or the earlier Australopithecus. The boy’s brain, which is estimated to be around 95% its projected adult size is only 420 cc, some 90 cc below the smallest brain known for early Homo (with a brain case of only 510 cc, KNM-ER 1813 itself is considerably smaller than other Homo specimens). It is on a par with the cranial capacity of the diminutive species Homo floresiensis.

The Malapa hominins have a mix of both australopithecine and Homo traits, with the authors of the paper suggesting greatest specific affinities to A. africanus. The small body, long arms and small brain case are indeed more suggestive of australopithecines. A. africanus, itself is a very variable species and it would not be absurd to suggest that the Malapa hominins represent one tail of the bell curve of variation within that species. The biggest difference between the Malapa hominins and A. africanus is the small dental dimensions of the former. Other traits are more typically associated with Homo, such as long legs, short hands, a derived pelvic configuration, gracile jaw with a weakly developed chin, small teeth, a flat face and a projecting nose. This mosaic anatomy should be a warning to palaeoanthropologists wishing to identify species based on a single anatomical feature.

It has been suggested that A. sediba could be a candidate ancestor for Homo, based on the number of derived traits it share with early representatives of that genus (more than any other known australopithecine). While the site is too late to be ancestral to Homo, the species may not be.

So should sediba be classified in the genus Australopithecus or Homo? The traditional way of distinguishing Australopithecus from Homo was the larger brain size of the latter (with a cutoff point of around 600 cc) and its use of stone tools. Using of a trait like brain size is highly problematic, since it is strongly correlated with body size and there is not a one-to-one correspondence between brain size and brain function. The recent discovery of H. floresiensis, with its small but derived brain, was found together with sophisticated stone tools. Similarly, a preliminary analysis of A. sediba suggests that its brain is more derived than its size would suggest. The first unambiguous appearance of stone tools in the palaeoanthropological record are attributed to H. habilis. Stone tools have not been recovered from Malapa but formal excavations have yet to get underway there. If stone tools are recovered it will require a rethinking about how we define our genus. While brain size is not the only distinguishing characteristic palaeoanthropologists use to separate Homo and Australopithecus, the dividing line is nonetheless an arbitrary one. For the moment, I think Australopithecus is a reasonable preliminary designation for this material, particularly considering our incomplete knowledge of the fossil record.

News headlines touting A. sediba as the “missing link” between humans and apes is misguided on multiple levels. The term “missing link” comes from an outmoded understanding of evolution. Moreover, humans did not suddenly appear with Homo. This is a gross over-simplification of how evolution works. We should not expect to see a momentous change between the first members of a new species or genus and their parent population. Indeed, there is considerable debate as to whether members of the species H. rudolfensis (e.g. KNM-ER 1470) and H. habilis (e.g. OH 24 a.k.a. “Twiggy”), which lie on the generic dividing line, would actually be more accurately classified as australopithecines. I’ve seen grown men (it seems to be men that get most bent out of shape about such technicalities) argue vehemently over such taxonomic subtleties. Evolutionary theory would dictate that the line between Homo and Australopithecines be a fuzzy one. In fact, if we had a complete fossil record it would be near impossible to know where to draw the line between different genera and species.

In the meantime, more individuals are being slowly uncovered at Malapa. Among these finds, are the arms bones of a 12 – 18 months old infant uncovered metres away from the two published specimens. Whether A. sediba maintains it australopithecine designation or not, is much less interesting than what this population tells us about hominin variation circa 2 million years ago.

References
Lee R. Berger, Darryl J. de Ruiter, Steven E. Churchill, Peter Schmid, Kristian J. Carlson, Paul H. G. M. Dirks, Job M. Kibii (2010). Australopithecus sediba: A New Species of Homo-Like Australopith from South Africa Science, 328, 195-204: 10.1126/science.1184944

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Taking a walk along our evolutionary trail

As the rain began to clear over the region a couple of hominins could be seen walking across the blackened earth. The sun was beginning to break through the mist, exploding in a rainbow of colours. The volcano had been making periodic muffled groans since the last eruption…

Some 3.6 million years ago the now extinct Sadiman volcano erupted in Laetoli, Tanzania. It released a plume of ash into the atmosphere. This was the rainy season and the rains changed ash into mud. Elephants, antelopes, hares, giraffes, pigs, rhinos, as well as some bird species walked over the muddied terrain. Among the footprints were those from a pair of (and perhaps even three) hominins, walking side-by-side. A second eruption released more ash into the air covering over the footprints, preserving them as a layer of tuff.

And so the it remained for more than three-and-a-half million years.

Mary Leakey sent an expedition to investigate Laetoli in 1974. One afternoon in 1976, a group of paleontologists were passing the time by throwing elephant dung at each other. Admidst the mud flinging, palaeontologist Andrew Hill found himself standing atop the now eroded ash layer. Archaeologists set about painstakingly excavating the footprints. The layer was friable and crumbled easily. After years of meticulous excavation, the footprints were exposed in all their glory; the grand prize being the fifty metre trail left by the hominins.

They are perhaps the clearest evidence for the early adoption of bipedal walking in our lineage. The footprints are thought to belong to Australopithecus afarensis, the species which included the famous fossil Lucy. However, there has been some debate as to whether these tracks represent fully bipedal locomotion or were more similar to the bent-knee, bent-hip gait seen when modern chimpanzees adopt a bipedal locomotion.

In a study that recently appeared in the journal PLoS ONE, human subjects were asked to walk over a specially constructed walkway. The surface of the track was covered with a damp sand, to mimic the soft underfoot condition that existed at Laetoli when the footprints were laid-down. The subjects walked twice across the trackway and then a further two times assuming a bent-knee, bent-hip gait. Walking with a normal modern human gait produced foot impressions with nearly equal heel and toe depths. In contrast, the bent knee gait resulted in footprints with deeper toe impressions than heel impressions. When non-human apes walk bipedally, weight is transmitted from the heel, along the outside of the foot, with toe-off occurring around the middle of the foot. We on the other transmit weight along the heel to the ball of the foot, finally toeing-off with the big toe. This is the more efficient way to walk bipedally. The impressions from Laetoli best match the pattern made by modern humans.

However I would be cautious about drawing too many conclusions from this study. One major drawback of this study is that walking with a bent-knee, bent-hip gait is not a natural gait for us. The impressions left by modern humans walking with this posture are probably not exactly the same as the footprints that a chimpanzee would leave when walking upright. While this study suggests that these hominins walked with a gait similar to our own, there is still room for debate as to exactly how similar the footprints are to our own. Regardless of these drawback, this study is a step in the right direction (no pun intended).

References
Raichlen, D., Gordon, A., Harcourt-Smith, W., Foster, A., & Haas, W. (2010). Laetoli Footprints Preserve Earliest Direct Evidence of Human-Like Bipedal Biomechanics PLoS ONE, 5 (3) DOI: 10.1371/journal.pone.0009769

The abrupt increase in brain size that wasn’t?

In an interview that recently appeared in the Guardian, neurobiologist Colin Blakemore has overstepped the mark in his discussion of the evolution of the human brain. There are a number of problems with Blakemore’s thesis that have been covered more than adequately by Jerry Coyne and John Hawks. I wish to focus on the claim that there was an “abrupt” increase in brain size in hominins around 200,000 years ago. Blakemore presents his argument as follows:

The question is: why is [our brain] so big compared to the brains of our predecessors, such as Homo erectus? Until 200,000 years ago, there had been a gradual increase in brain size among hominins, starting three million years ago. Then, abruptly, there was a remarkable increase of about 30% or so.

John Hawks is not convinced that there is any abrupt change in cranial capacity. Referring to the above graph showing endocranial volume against time he writes:

As you can see, there’s no sudden jump 200,000 years ago, or at any other time. The data, such as they are, are consistent with a single pattern of increase over time, as pointed out by Sang-Hee Lee and Milford Wolpoff (2003).

Heck, it’s the lack of a sudden jump that has gotten all the attention. Because if “modern” humans suddenly showed up in Africa 200,000 years ago, and all of a sudden had vastly larger brains than any other hominins, wouldn’t that be a simple and tidy story? Don’t you think we’d all be talking about the sudden origin of modern humans as reflected by their larger brains?

It just didn’t happen.

I decided to take the data from the Lee and Wolpoff paper and compare the periods prior and subsequent to 200,000 years ago. As Hawks eluded to, the data can be explained by a linear model. However, this is not very helpful since we can easily fit a line or curve to just about any data. More to the point, a single fitted line doesn’t tell us much about any changes in the data. The red line in the graph below corresponds to the best fit line for the entire dataset (r = 0.81). The green and orange lines are the best fit lines for the two time periods we are considering. We can see that slopes of all three lines differ appreciably from one another. An analysis of covariance test confirms that there is a significant difference in cranial capacity between the two time periods, after we control for time. The model is statistically significant: F(1, 84) = 107, p < 0.001.

Another way to consider our data is to look at the residuals. The residuals are simply the difference between our true values and the best fit line of our model. A good way to think about residuals is to imagine rotating our data above anticlockwise until the best fit line is horizontal. Since a horizontal line has a slope of zero, it also has a zero correlation with the x-variable, in our example time. In so doing, we can consider the differences in the residuals, having controlled for time. When we compare the residuals using the best fit line the means for the two time periods (separated by a grey dashed line) are significantly different. The model is also statistically significant: t(84) = -3.9994, p < 0.001. The mean difference in cranial capacity between the two periods is 122 cc; a difference of 31%. This corresponds well with Blakemore’s figure. However, it is important to note that this is the mean difference between the two periods and does not necessarily indicate an abrupt change at 200,000 years ago.

While the numbers seem to agree with the hypothesis of a marked increase in cranial size for the later period, I think the weight Blakemore gives it is rather foolish. The fossil record is patchy and likely unrepresentative of the true cranial variation of past hominins. As Jerry Coyne rightfully points out, a geologically sudden change in the fossil record may simply reflection how erratic it is. We already saw how cranial size can change markedly in 30,000 years – little more than a blip on the time scale that we are considering here. The gradual decrease in cranial capacity since the early Upper Palaeolithic would seem geologically sudden when considered on the above timescale. The size of the fossil record is small enough that the discovery of five or six new specimens could mean having to revise our figures once again.

Another problem is that calculating cranial capacity is not an exact science. While advances have been made in calculating cranial capacity, in many cases it should still be considered a best guess (de Miguel and Henneberg, 2001). This is particularly the case for palaeoanthropological material which tends to come out of the ground fragmented and deformed. With all its drawbacks, the fossil record is often all we have to answer some of our most pressing questions. At the same time, we need to always be conscious of what the record can and cannot tell us, and avoid the temptation to tell “fanciful tales”.

References

De Miguel C and Henneberg M (2001) Variation in hominid brain size: how much is due to method? Homo 52: 3–58.

Lee S-H and Wolpoff MH (2003) The pattern of evolution in Pleistocene human brain size. Paleobiology 29:186-196.

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