Man and Ape – an ancient hybrid population?

 "A Venerable Orang-outang", a caricature of Charles Darwin as an ape published in The Hornet, a satirical magazine, in 1871. This image is in the public domain as the photographer is unknown.

“A Venerable Orang-outang”, a caricature of Charles Darwin as an ape published in The Hornet, a satirical magazine, in 1871. This image is in the public domain as the photographer is unknown. Click image for source.

In 1865 in his book On the Origin of Species* Charles Darwin first described the idea of evolution by natural selection, suggesting that every biological population is subject to constant change. This change occurs via the influence of the environment in favouring traits which increase the likelihood of an organism surviving and reproducing. His proposal that humans and apes share a common ancestor triggered a famously heated debate amongst the general population. The “men from monkeys” view caused widespread indignation as it went against the idea that humans were a perfect creation of God. If we were indeed created, as the Bible says, in God’s image, we could not possibly have ancestors that looked like apes. The Bible definitely does not mention Adam or Eve looking like chimpanzees, after all.

Today plenty of data has accumulated in support of evolution by natural selection. Large number of scientists have tried to untangle the misinterpreted views around this topic, yet the embers of this debate still smoulder on – the theory of creationism is still believed by many (up to 40% of Americans), especially among those with religious views. Interestingly, many American schools teach creationism in biology classes. Some other countries also teach creationism but often the curriculum requires that evolutionary theory is taught – for example in Britain, students are presented with evidence for evolution, and the former Archbishop of Canterbury (Dr Rowan Williams, he of the luxuriant eyebrows) expressed the view that creationism should not be taught.

A few years ago it was feared that this 150-year-old debate was about to be fully reignited. A paper appeared on the horizon that scientists thought could potentially be twisted and used by creationists to argue their case! In 2006 Patterson et al published an article entitled ‘Genetic evidence for complex speciation of humans and chimpanzees’, which hypothesised that patterns of human and chimpanzee genetic diversity can be explained by the following scenario. In the beginning, only the ancestor of humans and apes existed, then 10 million years ago this ancestral population split into two isolated populations – one of these populations becoming the ancestor to humans and the other the ancestor to chimps.

Then, these now somewhat genetically different populations came back into contact and interbreeding between them created a hybrid (offspring of two animals from different species) population (confused yet?). Then to make matters even more confusing, this hybrid population then interbred back into either both or one of the parental populations, before a final separation between them occurred less than 6.3 million years ago, which led to formation of humans and chimpanzees as two distinct species.

But how can this information be misconstrued to support the creationist viewpoint? The fear was that creationists may argue that the fossils of the transition between humans and chimps are actually just the product of humans and chimps mating with each other to form a hybrid, so are not evidence for evolution. This would mean that their view that God created humans still holds.

Let’s look at the data in a bit more detail to examine how Patterson et al 2006 conclude that this hybridisation event happened (disregarding the possibility that they wanted to increase their chances of being published in Nature, as one way to potentially up the odds is to appear rather provocative and controversial – or so I have heard).

Patterson et al supplied three major arguments from the genetic data that, according to them, point to a hybridisation event. In the study, 20 million base pairs of genetic code from humans, chimpanzees, gorillas and more distantly related primates were aligned. In this technique you basically compare the genetic sequences of different species to see where they are similar or different. They used this to study the genetic differences and similarities between these species. This idea uses the fact that the longer two species have been separate species, the more differences you will find when their genetic sequence is compared. This means that species which have recently shared a common ancestor will be more similar than those who shared a common ancestor a long time ago. Their results suggest the following:

  1. There is a large percentage of genetic loci (simply ‘places’ in the genome) at which humans are more similar to gorillas than chimpanzees and where chimpanzees are more similar to gorillas than humans – 18-29% of the loci under consideration. This is really quite strange when you consider that humans and chimps are supposed to be the most closely related. As mentioned above, in general the more similar the genetic sequences of two species, the more recently they are predicted to have diverged from each other. This means that the authors found that different places in the genome predict a different order of divergence between humans, chimps and gorillas. They concluded from this that there must have been a hybridisation event.
  2. If you estimate how long ago chimps and humans split to form distinct species using just the similarities and differences in X chromosomes the divergence date is much more recent than that predicted by other chromosomes, which again the authors use to argue that hybridisation must have happened.
  3. So following the same train of thought, different parts of the genome suggest two different divergence dates all together. Some areas suggest that humans and chimps split 10 million years ago; others suggest it was six million years ago. Either way these dates suggest that the date of divergence predicted from genetic study is much earlier than that predicted from the fossil record, so the authors concluded that genetic divergence precedes species divergence due to this hybridization event.
  4. Additionally, they use the fossil of Sahelanthropus (a very old fossil of the subfamily Homininae,  which possibly dates to around the time of the split of humans from chimps – it is thought to be a member of the tribe Hominini, found in Chad) as evidence for their hypothesised hybridation. They do so by suggesting that it might be a representative of an intermediate species prior to the hybridization event as it is estimated to be around seven million years old; furthermore it possesses human-like features such as bipedalism (walking on two legs) and characteristic human dentition.

The paper did not escape criticism from the evolutionist perspective. One of the main attackers of the proposal was John D. Hawks, Professor of Anthropology at University of Wisconsin–Madison.  As he mentions, he expresses his views not because the theory suggested by Patterson really is of that much importance, as “the most likely result is that the rest of us scientists will just ignore or move past this specific hypothesis”, but because it might influence the views of the general public, via media outlets and their at times misleading coverage. His summary of the paper is as follows:

“I’ve read the paper, and I have to say it doesn’t deliver on its promises. It fails to cite previous work on the topic, it discards without explanation the hypothesis supported by most previous studies, and it promotes a “provocative” hypothesis for which there is no good evidence. It doesn’t even show that the speciation of humans and chimpanzees was “complex”. It’s just a mess.”

A very gentle, tactful comment; obviously not trying to hurt anyone’s feelings. He follows this blistering analysis with some specific examples of what might have been slightly misinterpreted by the authors.

The wide range of divergence times in human-chimpanzee genomes is not a groundbreaking result. Previous studies have estimated the divergence times to be between 5-7 million years and the time differs due to different parameters used in models of estimation, e.g. the effect of large effective population size (a measure of genetic diversity) of the population ancestral to humans and chimpanzees. Therefore the different divergence dates predicted by different regions of the genome could be just an effect of population history and population structure (for example, most populations contain subpopulations which may increase and decrease in size independently of each other and have different environmental pressures etc.) rather than the result of a hybridisation event. Different parts of the genome are subject to different selection pressures and therefore evolve at different rates. The authors fail to mention that divergence times they discover vary on a continuum from 10-6 million years ago (i.e. are not limited to two estimates of 10 and 6 million years ago) and fail to cite papers that discover that before them (Takahata, Satta, and Klein, 1995), Satta et al. (2004), Yang (2002)). Their results are therefore not new.

What is more, the divergence date of species is always estimated to be more recent when calculated using the X chromosome than when it is estimated using the nuclear genome. This is an effect of the type of inheritance the X chromosome experiences (females have two X chromosomes and males have one), so the evolutionary rate of the X chromosome is different than that of a nuclear genome, due to strong selection acting on it (which is stronger than on genes of the nuclear genome). This means that natural selection alone is sufficient to explain the result from the X chromosome analysis without including the hybridization event.

Last but not least, the paper fails to acknowledge the debate around the fossils of Sahelanthropus, which is considered by some as a female ape. The characteristics cited as ‘human’ are also very similar to those that distinguish male apes from female apes. What is more, bipedality in Sahelanthropus has not been firmly established, as the fossils that exist are damaged. In such fossils the key piece of damage is to the skull, which is one of the main pieces of evidence for bipedal walking. This is because the hole where the spine enters the skull (the foramen magnum) is further underneath the skull in bipedal animals than in non-bipedal animals.

Hybridisation is thus perhaps an unlikely scenario. What else is there to infer about evolution of humans from the genetic data? I am starting a Master’s research project on that next year so watch this space ;).

Also as far as I am aware creationists never tried to use this study as evidence for their views, so we can all calm down now.

To read more of John Hawk’s interesting thoughts please see here, or for a slightly more sympathetic review see here.

N. (Swedelicious Master’s Student)

Natalia * Full title On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, which surprisingly few people know, even Richard Dawkins once “couldn’t recall” what comes after ‘On the Origin of Species’.

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