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 surprisingly recently (right).
Copyright © 2018 MPI EVA/ S. Neubauer, Ph. Gunz
CC-BY-SA 4.0
CC-BY-SA 4.0
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.
Abstract
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.
INTRODUCTION
Our ancestors’ cognitive and behavioral abilities and the underlying brain morphology and function are critical for understanding the evolution of modern humans. Multiple lines of evidence from paleoanthropology, archeology, and genetics are informative about the evolution of brain and behavior in the Homo sapiens lineage, but there is no consensus about the tempo and mode of these biological and behavioral changes. In the absence of fossilized brains, we can study internal casts of the bony braincase. These endocasts approximate outer brain morphology because the brain, meninges, and cranial bones interact in an integrated and highly coordinated way during early development (1). Present-day modern humans have globular brains and globular endocasts with steep frontal, bulging parietal, and enlarged, rounded cerebellar areas. Together with small and retracted faces, this globularity characterizes the modern human skull (Fig. 1) (2,3). In contrast, Neandertals and other archaic Homo individuals have anterior-posteriorly elongated endocasts (4).Ontogenetic data show that the adult endocranial shape differences between H. sapiens and Neandertals develop prenatally or during a perinatal globularization phase found only in the former group (5–8). Developmental globularization therefore occurs in a period of high brain growth rates and is largely driven by the brain (1). A large body of literature including clinical evidence shows that the tempo and mode of brain growth during this period are related to neural wiring underlying brain function and behavioral capabilities (6, 7, 9–11). Developmental globularization leading to more globular brains in modern humans (6,7) and differences in early brain growth rates leading to slightly larger adult brains in Neandertals (12) are consequently interesting in the discussion of brain evolution and related behavioral changes. Endocranial shape changes during later ontogeny (that is, after the eruption of the deciduous dentition) are similar among present-day humans, Neandertals, and great apes with only some adjustments in the amount of shape changes (5–8,13,14). This shared segment of the ontogenetic pattern is thought to reflect interactions between the brain and the face, because the latter continues to grow after adult brain size has been achieved (5, 13–16).
Fig. 1 Differences in brain shape between a present-day human (left, in blue) and a Neandertal from La Chapelle-aux-Saints (right, in red).
Endocasts are shown together with the triangulated landmark set used in this study and CT scan renderings of the crania.
Hominin fossils from Jebel Irhoud (Morocco) that are associated with Middle Stone Age artifacts dated to around 300,000 years ago (17) display key features of modern human craniodental morphology including facial, mandibular, and dental characters comparable to later H. sapiens fossils or even present-day humans (18–20) as well as modern timing of dental development that suggests a human-like paced life history (21). Given these craniodental similarities, the Jebel Irhoud fossils are either interpreted as the currently earliest known members of the H. sapiens lineage (3,20,22) or as part of an ancestral population related to the origin of H. sapiens (23). However, the braincases of the Jebel Irhoud fossils are not globular (19,20,24). This demonstrates some independence of facial and neurocranial evolution in spite of important integration between these cranial modules via the cranial base (14–16). Together with other African fossils such as Omo Kibish [dated to around 195,000 years ago (25)], the Jebel Irhoud specimens force us to rethink the evolution of our species. Here, we therefore (i) investigate when and how the endocranial globularity typical of present-day modern humans emerged, (ii) analyze how this process is related to evolutionary brain size increase (4), and (iii) explore potential links between the evolution of the brain and genetic as well as behavioral changes.
Contrasting interpretations of the archeological record either see a rapid emergence of behavioral modernity at the transition to the Upper Paleolithic in Europe and the Later Stone Age in Africa possibly related to a mutation and consequently to neural changes (“human revolution” model) (26), or a gradual emergence as documented by the African Middle Stone Age without a specific biological correlate triggered by factors such as environmental changes or demographic developments (27). Features used to mark behavioral modernity range from worked bone, ornaments, pigments, and complex multicomponent lithic technologies to material indicators of manipulations of symbols and abstract thought such as unequivocal art. Some of those features are not exclusively known from modern human sites, and others are documented systematically only since the Upper Paleolithic (28).
Ancient DNA of archaic Homo representatives and H. sapiens fossils revealed derived genetic features that were fixed in H. sapiens after the population split from the clade including Neandertals and Denisovans more than 500,000 years ago (29–33). These genetic data suggest positive selection within our lineage on genes important for brain function and behavior and, especially, the development of the nervous system [for example, genes involved in axonal and dendritic growth or synaptic transmission including NOVA1, SLITRK1, KATNA1, LUZP1, ARHGAP32, ADSL, HTR2B, and CNTNAP2 (30)]. Another example is FOXP2, a gene that is important for normal development of speech and language. Although amino acid substitutions specific to modern humans were also found in Neandertals, one substitution in an intron of this gene that affects a binding site for a transcription factor and likely alters the regulation of FOXP2 expression and associated behaviors is absent or polymorphic in Neandertals (31). On the other hand, a recent analysis showed that Neandertal genetic material that introgressed in the modern human lineage affects cranial and brain morphology of present-day humans (34).
Here, we analyzed endocasts of H. sapiens fossils from different geologic time periods. Previous quantitative analyses used smaller samples and were based on the endocranial midsagittal plane only (), external landmarks only on the posterior neurocranium (19), or a set of some endocranial landmarks (4,24). Here, we used geometric morphometrics based on three-dimensional coordinates of endocranial landmarks and hundreds of curve and surface semilandmarks (5,35–37) measured on computed tomographic (CT) scans (fig. S1). We obtained landmark data for 20 H. sapiens fossils (Table 1) that can be divided into three groups according to geologic ages: (i) early H. sapiens from North and East Africa that lived about 300,000 to 200,000 years ago and therefore document the morphology of the currently earliest known representatives of our clade since the population split with Neandertals, (ii) Levantine and East African individuals that lived about 130,000 to 100,000 years ago, and (iii) Upper Paleolithic and geologically younger individuals that are about 35,000 to 10,000 years old. We compiled the same data for comparative samples of cranially diverse present-day humans from all over the world and archaic Homo representatives (Neandertals, Middle Pleistocene H. heidelbergensis sensu lato, and H. erectus sensu lato, as listed in Table 1) and computed Procrustes shape variables (see Materials and Methods). Incomplete and distorted fossils were reconstructed using established methods of computer-assisted paleoanthropology [see Materials and Methods and previous studies (36,38,39)].
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)
Note 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 in 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 explains 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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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."
ReplyDeleteSorry Geisler, but you're wrong!
IOW, a classic 'God of the Gaps' fallacy.
DeleteThanks
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