A few weeks ago, I ran a discussion group hoping to discuss the primary cause of the Pleistocene megafaunal extinction event. Archaeological evidence suggests that up to 85% of large fauna weighing over 44kg went extinct on several continents and many islands towards the end of the epoch. Given the timings of these events, two major, competing hypotheses still rage on in the literature today, almost with ideological fervour. The first hypothesis is that the extinction of these fauna coincide with the retreat of the Last Glacial Maximum (LGM), and that many of these larger, already food-limited species were simply unable to adapt to the warming climate. The second hypothesis places us at the centre of the scene: the bloody culprit, Homo sapiens and predecessors thereof. Proponents of this “overkill hypothesis” argue that the last known fossils of these extinct genera and the first known evidence of human appearance coincide in the fossil record in each of these locations too often for it to be a coincidence. One of the reasons why the argument has become controversial is that some people have begun to question our moral requirement to pay ecological reparations of kind. If humans were indeed the primary cause behind such a large extinction event, do we feel an obligation to make amends?
So we mused and puzzled over these lines of evidence, trying to determine within the short space of an hour whether one was more convincing than the other. However, shortly into the discussion, there came one point in which we became quite unstuck. Instead of weighing up the data as if they were equal, we started wondering; how can anyone be sure of causation using archaeological evidence at all? It was quite unlike the data sets that we are used to. So, say you are digging in the Yukon Territory in Canada and you come across several sets of equid fossil species spanning several thousand years, clearly these horses are getting smaller over time you observe, clearly humans weren’t in this region at the time you think, clearly indicative of environmental pressure of the glacial retreat you surmise, clearly reflective of the climate change hypothesis you conclude. Excellent. A nature paper awaits! But wait. It is likely that you may have committed quite a few of the major issues associated with extrapolating information from fossil data in your haste.
Firstly, you are likely suffering the plague of all paleo-scientists: taphonomic bias. Taphonomic bias is the process by which we are only privy to the type of data that can inherently survive throughout geological time. Soft materials, smaller fauna and events that took place in raging rivers are less likely to be preserved than their counterparts, leading to the under- or over-representation of certain taxa or events. Whole species, whole ecosystems could have occurred during this time, ones that we may never even know they are missing. A potentially very significant source of taphonomic bias that we came across during our discussion was the timing of the first hominid arrival into North America. The current general consensus is that this fell somewhere between 13,000 and 20,000 years ago. However, there are some pieces of evidence that suggest it could even be as far back as 50,000. The difference in the upper and lower ends of the estimate could have significant consequences for how much blame we can attribute to humans for the extinction event. Humans that arrived only 13,000 years ago were indeed quite pushed for time when trying to condemn equid species to extinction. Of course you only have to look back to see that the presence of a rather large ice sheet over North America until about 20,000 years ago could have had significant impacts on whether we would be able to detect their presence beforehand.
Another pitfall when trying to make conclusions based on this type of data is that it is impossible to conduct experiments or make observations because there are no factors to manipulate and no events to watch. Determining causation becomes, at best, an educated guess and using contemporary situations as a stop-gap can be no more illuminating. In his paper, Guthrie (2003) states that past equids were obligatory grazers based on the assumption that modern-day horses can be used as a proxy measure for their predecessors feeding habitats. However, using modern-day analogies to try to extrapolate into the past can also be very misleading. Contemporary equids are adapted to a different climate, different predators and by now a range of anthropogenic factors.
The one thing that could help to mitigate these issues – a plethora of data – is also seldom available when using taphonomic data. Given how difficult it is for data to make it through the battering of time, it is not all that surprising either. In the figure above, the sparsity of measurements has meant that the author has made statistical inference about this relationship based on one data point over a 10,000 year time period. Without this solitary data point, would the relationship still hold true? Would the timings of events still line up in a way that supports his argument?
So assault after assault, how do these questions ever get answered? It seems almost overwhelming and we certainly weren’t going to solve it in the hour that we had. What we did get, however, was an insight into a surprisingly dynamic field for an event which may or may not have even occurred when humans were around to witness it.
Dale Guthrie, R. (2003). Rapid body size decline in Alaskan Pleistocene horses before extinction. Nature, 426(6963), pp.169-171.
Ripple, W. J., & Van Valkenburgh, B. (2010). Linking top-down forces to the Pleistocene megafaunal extinctions. BioScience, 60(7), 516-526.
Soligo, C. and Andrews, P. (2005). Taphonomic bias, taxonomic bias and historical non-equivalence of faunal structure in early hominin localities. Journal of Human Evolution, 49(2), pp.206-229.