neurosciencestuff: (Image caption: Brain shape evolution in…


(Image caption:
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).
© MPI EVA/ S. Neubauer, Ph. Gunz)

Modern human brain organization emerged only recently

The evolutionary history of our own species can be traced back to fossils from Jebel Irhoud (Morocco)
dated to about 300,000 years ago. Last year’s analysis of these fossils
by researchers from the Department of Human Evolution at the Max Planck
Institute for Evolutionary Anthropology in Leipzig was highlighted as
one of the top science stories of 2017 by a diverse range of print and
online media. Together with crania from Florisbad (South Africa, 260,000
years old), and Omo Kibish (Ethiopia) dated to 195,000 years ago, the
Jebel Irhoud fossils document an early evolutionary phase of Homo sapiens
on the African continent. Their face and teeth look modern, however
their elongated braincase appears more archaic as in older human species
and in Neandertals. In contrast, it is a globular braincase, which
characterizes the skull of present-day modern humans together with small
and gracile faces.

In a new paper published in Science Advances, members of the same
research team now reveal additional surprising findings about brain
evolution in Homo sapiens. The paleoanthropologists Simon
Neubauer, Jean-Jacques Hublin and Philipp Gunz used micro computed
tomography scans to create virtual imprints of the internal bony
braincase, so called endocasts that approximate brain size and shape.
They used state-of-art statistics to analyze endocasts of various
fossils and present-day humans.

Evolution of the parietal lobe and the cerebellum

Neubauer and colleagues document a gradual change within Homo sapiens,
from an elongated endocranial shape towards a more globular one. Two
features of this process stand out: parietal and cerebellar bulging.
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.

The Homo sapiens fossils were found to have increasingly more
modern endocranial shapes in accordance with their geological age. Only
fossils younger than 35,000 years show the same globular shape as
present-day humans, suggesting that modern brain organization evolved
some time between 100,000 and 35,000 years ago. Importantly, these shape
changes evolved independently of brain size — with endocranial volumes
of around 1,400 milliliters, even the oldest Homo sapiens fossils
from Jebel Irhoud fell within present-day variation of brain size. “The
brain is arguably the most important organ for the abilities that make
us human,” says Neubauer. But modern human brain shape was not
established at the origin of our species together with other key
features of craniodental morphology. Neubauer adds: “We already knew
that brain shape must have evolved within our own species, but we were
surprised to discover just how recent these changes to brain
organization were.”

Evolutionary changes in early brain development

In present-day humans, the characteristic globular shape of the
braincase develops within a few months around the time of birth. Philipp
Gunz explains, “The evolution of endocranial shape within Homo sapiens
suggests evolutionary changes of early brain development – a critical
period for neural wiring and cognitive development.” The researchers
therefore argue that evolutionary changes to early brain development
were key to the evolution of human cognition. Jean-Jacques Hublin,
co-author and director of the Department of Human Evolution at the Max
Planck Institute in Leipzig, says: “The gradual evolution of modern
human brain shape seems to parallel the gradual emergence of behavioral
modernity as seen from the archeological record.”

The new findings are in agreement with recent genetic studies that
show changes in genes related to brain development in our lineage since
the population split between Homo sapiens and Neandertals. They add to the accumulating archeological and paleoanthropological evidence demonstrating that Homo sapiens
is an evolving species with deep African roots and long-lasting gradual
changes in behavioral modernity, brain organization, and potentially
brain function.

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