Charles Darwin
born 12th Feb 1809

A Question of Science

"The issue: the idea that human evolution was triggered by an aquatic phase is widely supported and discussed outside the mainstream of paleoanthropology, but is dismissed by most scientists working in the field. How do we determine what models are reasonable and plausible, and which ones are worthy of serious scientific study?"

- from Principles of Human Evolution - Roger Lewin & Robert Foley, Oxford, Blackwells, 2003

The 2009 £500 PRIZE for the best response to this question in under 2,500 words by a student in the UK was won by
ALEXANDER CAGAN of St Catharine's college, Cambridge

Sponsored by Eildon Press

WINNING ENTRY

"The issue: the idea that human evolution was triggered by an aquatic phase is widely supported and discussed outside the mainstream of paleoanthropology, but is dismissed by most scientists working in the field. How do we determine what models are reasonable and plausible, and which ones are worthy of serious scientific study?"

"The men of experiment are like the ant, they only collect and use; the reasoners resemble spiders, who make cobwebs out of their own substance. But the bee takes the middle course: it gathers its material from the flowers of the garden and field, but transforms and digests it by a power of its own. Not unlike this is the true business of philosophy (science)" (Francis Bacon 1620).

Everybody likes a good story and, in the case of human evolution, many people expect one. The potential and the temptation for narrative-driven story-telling is, perhaps, nowhere greater than in the field of human evolution. Origin myths would appear to be a universal of all human societies and those who seek to study and better understand human evolution must be careful not to end up telling such stories themselves, equally speculative but given credibility by a thin veil of pseudoscience. The only way to gain true knowledge about human evolution is through rigorous use of the scientific method. This involves the formulation of hypotheses that can be repeatedly tested and falsified, using evidence that is both empirical and observable. For a model to be worthy of serious scientific study it must meet these criteria, otherwise it is in danger of becoming another example of what Stanford and Allen (1991) call "strategic storytelling"; these are models which appear to be driven by sound scientific principles but which are in fact un-testable and therefore not truly scientific. However, this is bound to be difficult because the study of human evolution is not aimed primarily at discovering underlying evolutionary principles that can be tested. The study of human evolution requires a further step in which we use such principles in order to understand, as fully as possible, what actually happened to our ancestors (and non-ancestors!) in the past. As our ancestors and the environments in which they evolved no longer exist we must use indirect means to determine all the whats, whens, wheres, hows  and whys of our evolution. The scarce indirect evidence that survives in the form of fossils, archaeological remains and genetics provide invaluable insights into human evolution, but evidence alone is silent and is given meaning only through the context of the models we create. There are many ways that models concerning human evolution can be constructed, and by discussing the potential and the pitfalls of different types of models, it will become apparent that whatever approach a model takes it must be based on the principles of the scientific method for it to be worthy of serious study.

First and foremost models of human evolution must be rooted in a paradigm of evolutionary theory. Evolutionary theory provides the only scientific paradigm for understanding the change of natural organisms through time. However, to accurately apply evolutionary theory to human evolution it would, at first, seem necessary to know what our human ancestors and the environment in which they existed was like, and how they interacted with and within this environment. It appears to be a chicken-and-egg situation. Fortunately, because evolutionary theory is based on simple and constant principles, it is possible to extrapolate theories gained from the observation of other extant species and create models, which can then be applied to human evolution. This theory of uniformitarianism, which assumes that natural processes operating today are identical to those that operated in the past, is what allows us to create evolutionary models of human evolution, indeed it was this theory of uniformitarianism, developed by Charles Lyell (1830), which in part inspired Darwin to develop his theory of evolution by natural selection.

The ways that evolutionary theory is incorporated into models of human evolution can be either direct or indirect. Explicitly conceptual models take as their basis pure evolutionary theory and then attempt to use logical steps to conclude how such theory would have affected our ancestors. An example of this is the theory of anisogamy, which has been used to explain sexual dimorphism and mating strategies in sexually reproducing organisms. The theory goes that as female gametes (eggs) are rarer and more expensive to produce than male gametes (sperm), females should be more choosey about who they mate with as fertilisations that do not result in offspring who survive to reproduce are evolutionarily very costly, while males should mate with whoever they can as such mistakes are relatively inexpensive for them. This then results in sexual selection and sexual dimorphism as the males are selected for their ability to compete for females (usually through being the biggest and strongest) while females are selected for their ability to choose the best mates (Mayr 2001). Therefore, in species where males have most to gain by being competitive, such as polygynous species, we can expect to see a high degree of sexual dimorphism, while in species where the evolutionary stakes are lower so to is the extent of sexual dimorphism. Using this theoretical framework evolutionary anthropologists have attempted to model the reproductive systems of extinct hominins (Reno et al 2005).

While many see the direct and logical use of evolutionary theory in such non-referential models as a source of strength; arguing that they are not obscured by the smoky lens of unsuitable homologies and analogies, such a reliance on theory can also cause major problems. Although evolution is based on simple principles in reality it can be incredibly complicated and simple logical sequences often fail to take into account external factors that have not been considered. For instance the theory of anisogamy and its "logical consequence" of sexual dimorphism reflecting the mating system was initially interpreted as predicting that species with no marked sexual dimorphism would be monogamous. However, this has been questioned due to observational data of genera such as the gibbons, which have almost no sexual dimorphism but are not fully monogamous, with some groups being multi-male and many instances of extra pair copulations being observed (Barelli et al 2008). Revelations such as these are the result of observational data and show the inadequacy of theory alone in modelling human evolution. Referential modelling should not be seen as a necessary evil due to our lack of a direct window onto human evolution but as an opportunity to strengthen our understanding of the evolutionary theory on which models of human evolution are based and therefore stimulating new avenues of research. That said, for referential models to be of use they must use a relevant reference point. For example, if we were to try and model the origins of tool-use in early hominins it would probably be wiser to choose the tool-using Pan troglodytes rather than the non-tool using (in the wild) Pan paniscus as our referential model, although both are of equal degrees of relatedness to modern humans. Referential models can be based on a variety of factors, from phylogeny, where we use our closest living relatives, to ecological similarity, where we chose a species based on it sharing a similar environment to our ancestors. Therefore it is wrong to say that a referential model based on human hunter-gatherers is always better than one based on baboons simply because the human model is clearly more homologous. The relevance of a referential model depends on the specific question being asked.

While all models considered worthy of serious scientific study should integrate evolutionary theory with appropriate use of referential models they must also be capable of generating hypotheses that can be empirically tested and falsified. This ability to generate testable hypotheses is probably the key distinction between credible and non-credible models of human evolution. Many theories of human evolution that have gained popularity, such as the aquatic ape hypothesis (Hardy 1960, Morgan 1999) are almost impossible to falsify (Langdon 1997). The aquatic ape hypothesis is one of many models of human evolution popular among non-anthropologists, which relies on a single causal force, in this case our ancestors spending a brief period adapting to a littoral environment, to explain the majority of consequences in human evolution. Such models may artificially appear to be the most parsimonious theories of human evolution because a misunderstanding of the principle of Occam's razor leads people into believing that a model that can explain all of human evolution using only one cause is more likely to be correct than explanations of human evolution which resort to using multiple models to explain multiple factors. In reality an umbrella hypothesis that appears to be parsimonious in its explanation of multiple factors is in fact not parsimonious at all if it cannot be subjected to empirical testing and potentially falsified. From what we do know of human evolution it is exceedingly unlikely that single causal factors have such a wide scope for explanation, as the evidence we have suggests that human evolution was a mosaic process, both spatially and temporally. For example Langdon (1997) has shown that a range of separate causes, which are more amenable to the scientific method, can actually better explain most of the consequences explained by the aquatic apes hypothesis.

The scientific reality of the study of human evolution is not as immediately attractive as umbrella models because it does not easily take the form of a cohesive story. Progress in the study of human evolution depends on well thought out theories based on the study of what we have available to us in the present, namely referential models, which can be broken down into testable hypotheses. We need evolutionary theory to know which referential model is most relevant to use and we need referential models to help develop and refine evolutionary theory and to alert us to the diverse possible outcomes of evolution, which we alone would have difficulty imagining. We must both widen our eyes and focus our minds. While broader models have more appeal, it is the more specific and empirical ones with testable assumptions that will stimulate research and therefore further our knowledge and the discipline.

REFERENCES

Bacon, F. (1620) The New Organon. Cambridge UP.

Barelli, C, Heistermann M., Boesch, C., Reichard, U.H. (2008) Mating patterns and sexual swellings in pair-living and multi-male groups of wild white-handed gibbons, Hylobates lar. Animal Behaviour 75:991-1001.

Hardy, A. (1960) Was man more aquatic in the past? New Scientist 7: 642-645.

Langdon, J.H. (1997) Umbrella hypotheses and parsimony in human evolution: A critique of the Aquatic Apes Hypothesis. Journal of Human Evolution, 33:479-494.

Lyell, C. (1830) Principles of Geology. John Murray, Albemarle-Street.

Mayr, E.  (2001) What Evolution Is. London: Phoenix.

Morgan, E. (1999). The Aquatic Ape Hypothesis. Souvenir Press.

Reno, P.L. et al. (2005) The case is unchanged and remains robust: Australopithecus afarensis exhibits on moderate skeletal dimorphism. Journal of Human Evolution, 48:279-288.

Stanford, C.B. & Allen, J.S. (1991) On strategic storytelling: Current models of human behavioural evolution. Current Anthropology, 32 58-61.

Prof. Foley's comments

Humans are clearly a complex and unique species, and attempts to understand them evolutionarily and scientifically have come from all directions. The problem is not too few explanations, but too many. The difficulty lies in narrowing down the possibilities in ways that can inspire good science. Cagan's essay recognises this, and he provides an outline of how one should develop models that are consistent with biological principles and provide testable predictions. To a large extent this provides the answer to the question posed - fit with evolutionary theory and testability. The essay is a worthy winner for addressing the question and providing examples that support his argument.