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A Mosaic Evolution

Contemplate these two important points:

· There is no linear evolutionary history of the human species.

· Different branches of our evolutionary tree coexisted. And often intersected.

In short, we need new pictures in our head.

You know the “March of Progress” illustration?

Of course you do. We all do. It’s ubiquitous.

The illustration showed the alleged “evolution” from Pliopithecus (22-12 million years old) to Neanderthal (100,000-40,000 years old), Cro-Magnon Man (40,000-5,000 years old) and Modern Man, 40,000 years-present) with 11 other stops in-between.

Forget it.

A study by the University of Barcelona, published in the journal Scientific Reports and now garnering buzz, has broadened our thinking of how Homo sapiens came to look the way they do today. (A much more reader-friendly recap of the research is here at the Technology Networks website.)

To be clear, we know this whole idea of ‘mosaic evolution’ is not brand spanking new—but more and more hard evidence is coming to light.

The study analyzed mutations that are common in modern humans, but not in other species of archaic humans.

One big moment (we use the term loosely, since it’s a hard time to pinpoint) came about 40,000 years ago. This was a time associated with the emergence and growth of the Homo sapiens populations and departure from Africa. Another moment came about 100,000 years ago at a point when Africa was home to diverse types of Homo sapiens.

The researchers zeroed in on genetic variants related to behavior and facial structure. Why? Because behavior and facial structure are both fundamental traits that differentiate our species from other human species. The study also analyzed variants related to the brain. In particular, they looked at the volume of the cerebellum and corpus callosum.

This quote from a researcher (the effort was headed up by a University of Barcelona PhD student by the name of Alejandro Andirkó) sums up the story:

"The breadth of the range of human diversity in the past has surprised anthropologists. Even within Homo sapiens there are fossils, such as the ones I mentioned earlier from Jebel Irhoud, which, because of their features, were thought to belong to another species. That's why we say that human beings have lived a mosaic evolution.”

The fossils from Jebel Irhoud (that’s in Morocco) suggest a species that was a half-step away from being a full Homo sapien. The site includes animal remains and stone tools and dates to about 350,000 years ago.

The methodology used in the study was based on something called Genealogical Estimation of Variant Age method. That method was developed by researchers at the University of Oxford.

Says Andirkó, "the understanding of the deep history of our species is expanding rapidly. However, it is difficult to determine when the genetic variants that distinguish us from other human species emerged. In this study, we have placed species-specific variants on a timeline. We have discovered how these variants accumulate over time, reflecting events such as the point of divergence between Homo sapiens and other human species around 100,000 years ago.”

In short, evolution is not and was not on a straight line. The structures for bipedal locomotion occurred, at first, without any change in skull form or brain size. Later, both skull and brain evolved rapidly into the state of development associated with modern human species. (That’s from Brittanica).

In other words, natural selection acts differently upon the various structures and functions of evolving species. Evolutionary pressures for upright posture were a higher priority over the need for a complex brain. The elaboration of the brain, in turn, was probably linked to the freeing of the forelimbs made possible by going bipedal.

Thanks to all the paleoanthropologists who continue to explore and probe these fascinating questions. To sum it up, our “whole” animal today is the result of many pieces and parts. We are a mosaic.






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