January 19, 2016

“Virtual paleontology” provides insight into last common ancestor with Neanderthals

by Prof. Nathan H. Lents

Cross-posted with the permission of Prof. Nathan H. Lents, originally posted at The Human Evolution Blog.

While we are still discovering how and where the many branches of the hominin family tree explored the world, most evidence points to Africa as the place where the H. sapiens lineage evolved into our current form. At the same time as H. sapiens was developing, Neanderthals were evolving from their ancestors in Europe and Western Asia. There were clear differences in morphology (physical structure) between the two groups: Neanderthals were shorter, more thickly built, with bowed legs and strong upper bodies. They also had a very prominent brow ridge and a protruding lower face.

But the H. sapiens and Neanderthals also evolved some things in common. This is called parallel or convergent evolution and can sometimes confuse our normal process of studying how and when features evolved. (See the Visionlearning module Charles Darwin III: Descent with modification to learn more about the process of evolution.) In the case of humans and Neanderthals, both evolved increasing cranium size and fine hand-finger motor skills, probably as a result of tool use.

The Australian and North American moles appear very similar and have many shared features. Nevertheless, they are very distantly related, the former being a marsupial, the latter being placental.

Usually, when two closely related species share a feature, we assume that it was inherited from a common ancestor that also had that feature. Every so often, that’s not the case. Similar selective pressures can lead to similar adaptations and so physical likeness is not always the result of shared ancestry. For example, both bats and birds have wings, but that’s not because they evolved from a common ancestor that had wings. The wings of bats and birds are examples of convergent evolution.

Sorting out which features common to humans and Neanderthals are the result of shared ancestry and which are the result of parallel evolution can be very tricky. Sometimes, we simply don’t have all the fossils we need in order to make the determinations definitively. Paleoanthropologists estimate that the human and Neanderthal lineages diverged from their common ancestor between 350-400,000 years ago. This estimate comes from a variety of techniques, especially the “molecular clock” method which utilizes the known and stable rate of mutation in certain non-coding regions of DNA.

In order to address the question of what the ancestors of humans and Neanderthals might have looked liked, a research group at the University of Cambridge took a digital approach. They catalogued a large set of morphometric variables in the hominin fossil record, focusing on skull features. (“Morphometrics” are the measurements of variation in some feature.) The analysis included not just human and Neanderthal skulls, but representatives from many other populations along the way.

Figure 2 from Mounier & Lahr, J. of Human Ev. 2016.

The power of this technique is that even fragmentary fossils can be used (and they’re pretty much all fragmentary when we’re talking about the human fossil record). Any fossil that can provide data for even just a single measure can add something to the analysis, as long as its approximate age is reliably known and it can be placed within a species with other fossils. Using three-dimensional image analysis, the researchers analyzed the plots of the skull features simultaneously and then projected how they changed in the various Homo lineages. They aligned the 3D morphometrics of the human and Neanderthal lineage to see where they most overlap. This overlap point represents the morphometric state at which the human ancestor was most similar to the Neanderthal ancestor.

Because the fossil record is always an imperfect scatter of information, there are gaps in our knowledge, and the researchers were faced with at least three points in the historical record at which the common ancestor might have existed. In other words, their were three “models” that could have explained when the divergence of sapiens and Neanderthals might have occurred. Ultimately, the researchers were able to generate a hypothetical composite image of what the skull of the common human/Neanderthal ancestor might have looked like. Here it is:

Photo credit: Aurélien Mounier, University of Cambridge

The generation of this composite skull represents an emerging phenomenon: virtual ancestor reconstruction. This new technique has the mind-blowing power to generate and 3D-print images of predicted fossils that we can all see and handle. The value of this in evolutionary research and education is unprecedented as it “creates” fossils that scientists have only speculated about until now.

 

Prof. Nathan H. Lents

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Professor Nathan H. Lents is a tenured associate professor of molecular biology at John Jay College of the City University of New York, a visiting professor at the University of Lincoln (UK), and author of "Not So Different: Finding Human Nature in Animals" (Columbia University Press, 2016). Professor Lents conducts research in three areas: forensic botany, the human microbiome, and teaching/learning biology at the college level. His work has been funded by the NIH, NSF, US Dept. of Ed, and the Susan G. Komen Breast Cancer Foundation. He also maintains The Human Evolution Blog and authors most of its content.