Wednesday, 31 January 2018

Evolutionary Transition To Modern Humans

Brain shape evolution in Homo sapiens: brain shape of one of the earliest known members of our species, the 300,000 year-old cranium Jebel Irhoud 1 (left). Brain shape, and possibly brain function, evolved gradually. Brain morphology has reached the globularity typical for present day humans suprisingly recently (right).

Copyright © 2018 MPI EVA/ S. Neubauer, Ph. Gunz
License: CC-BY-SA 4.0
Modern human brain organization emerged only recently | Max Planck Society

The fossil record shows a distinct gradual transition to the modern human brain and cranial capacity from more ape-like origins, even within the species we recognise as Homo sapiens.

This is the finding of researchers from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, according to an open access paper published a few days ago in Science Advances. The finding also illustrates nicely how science incorporates new information, even information that at first glance doesn't seem to fit what we thought we knew, and how that new information can lead to a revised understanding and an improved explanation.

Modern humans have large and globular brains that distinguish them from their extinct Homo relatives. The characteristic globularity develops during a prenatal and early postnatal period of rapid brain growth critical for neural wiring and cognitive development. However, it remains unknown when and how brain globularity evolved and how it relates to evolutionary brain size increase. On the basis of computed tomographic scans and geometric morphometric analyses, we analyzed endocranial casts of Homo sapiens fossils (N = 20) from different time periods. Our data show that, 300,000 years ago, brain size in early H. sapiens already fell within the range of present-day humans. Brain shape, however, evolved gradually within the H. sapiens lineage, reaching present-day human variation between about 100,000 and 35,000 years ago. This process started only after other key features of craniofacial morphology appeared modern and paralleled the emergence of behavioral modernity as seen from the archeological record. Our findings are consistent with important genetic changes affecting early brain development within the H. sapiens lineage since the origin of the species and before the transition to the Later Stone Age and the Upper Paleolithic that mark full behavioral modernity.

Simon Neubauer, Jean-Jacques Hublin and Philipp Gunz
The evolution of modern human brain shape
Science Advances
24 Jan 2018: Vol. 4, no. 1, eaao5961 DOI: 10.1126/sciadv.aao5961

Copyright: © The authors
Published open access
Reprinted under a Creative Commons Attribution-NonCommercial 4.0 International license (CC BY-NC 4.0)

Not how the team have assimilated the information that the remains of early modern humans had been found at Jebel Irhoud, Morocco which were believed to be 300,000 years old, when we thought that modern humans didn't emerge until about 200,000 years ago. Now we know two things: that modern humans emerged from a more widespread species than we thought with some populations in North Africa and they had evolved into near anatomically modern humans at least in this population apart from having an elongated cranium, a more robust face and a brain that was structurally subtly different to modern humans. Note that these early H. sapiens had a cranial capacity that fell within the range of modern humans; what changed was the shape and so the organisation of the brain.

The thinking now is that modern humans are the result of diversified populations evolving for long periods in isolation then later recombining when they met again, similar to what we now think happened when modern humans came into contact with the descendant of earlier migrations out of Africa - Neanderthals, Denisovans and maybe others.

What this has given palaeoanthropologists is a 300,000 year long series of humans as they transitioned from the earliest archaic forms to today's humans. Together with fossils from, south Africa (260,000 years old), and Omo Kibish, Ethiopia (195,000 years ago), the Jebel Irhoud fossil gives us a picture of the early evolution of H. sapiens on thein African. These early members of the species had the elongated crania seen in archaic hominins and Neanderthals.

The change has been a gradual one from elongated to more globular and with it the parietal and cerebellar regions have bulged. According to the Max Planck Institute press release which accompanied the paper:

Parietal brain areas are involved in orientation, attention, perception of stimuli, sensorimotor transformations underlying planning, visuospatial integration, imagery, self-awareness, working and long-term memory, numerical processing, and tool use. The cerebellum is not only associated with motor-related functions like the coordination of movements and balance, but also with spatial processing, working memory, language, social cognition, and affective processing.

Perhaps significantly as an explanation for recent human history, only fossils younger than 35,000 years show the typical modern globular shape. This is significant in that it offers an explanation for what appears to have been a quite sudden increase in cognitive abilities in modern humans leading to language, agriculture, urbanisation and above all, learning and reasoning abilities and probably artistic and aesthetic appreciation as displayed in cave and rock paintings.

So we now have a description of the evolution of our species from primitive to advanced over the space of about 250,000 years and how physical evolution led to behavioural changes.

Yet another example of the immense explanatory power of evolution theory and another example of the evolutionary transition and progress that creationists insist never happened because they have an old book that tells a story of magic written by people who thought Earth was flat and had a dome over it.

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1 comment :

  1. This was a great read, esp. so as the first thing I read this morning was some Christian apologetics about the (purported) lack of gradualism in the fossil record. Norman L. Geisler says in his Big Book of Christian Apologetics, "...the gap between a primate and a human brain is immense. And this gap does not refer merely to the size of the brain but to its complexity and ability to create art, human language, and highly complex mechanisms."
    Sorry Geisler, but you're wrong!


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