16 | New Scientist | 28 September 2019
UNTIL recently, the only evidence
for the existence of a mysterious
group of ancient humans known
as the Denisovans was ancient
DNA extracted from a finger bone
and three teeth found in the
Altai mountains in Siberia. Now
a team has created a portrait of a
young Denisovan woman based
on that finger bone DNA – but
other researchers are sceptical
of the method.
Like Neanderthals, Denisovans
are an extinct type of human
that interbred with Homo sapiens.
But we still know little about them.
Liran Carmel at the Hebrew
University of Jerusalem in Israel
and his colleagues have used
Denisovan DNA to generate a
portrait that roughly represents
what Denisovans looked like. “Our
reconstruction is generalistic,”
says Carmel. “We just reconstruct
the face of the human group, not
of a specific individual.”There has long been interest in
working out what people look like
on the basis of their DNA alone,
for instance to help identify
suspects from a crime scene.
But our appearance depends
on thousands of variants in gene
sequences, each of which usually
has only a tiny effect. “Today we
cannot predict very much about
a person’s bone morphology,”
says John Hawks at the University
of Wisconsin-Madison.
Instead of looking at the DNA
sequence of genes to predict
appearance, Carmel and his
colleagues looked at how activethese genes were. When genes
get switched off in cells, epigenetic
tags called methyl groups are
added to their DNA. The team
has developed a way to identify
where these tags have been
added to ancient DNA.
Carmel’s group compared
the methylation patterns in
the ancient finger bone to those
in bone cells from modern
humans and chimpanzees,
revealing thousands of genes
whose activity was probably
different in Denisovans. Next,
the team tried to identify which
of those changes would affect
bone shape, based on what
happens when mutations disable
these genes in modern humans.
Finally, the group applied these
findings to infer how the growth
of Denisovan bones may have
differed from ours. The method
tells us in what way the bones
differed but not by how much,says Carmel. For instance, it
suggests Denisovans had wider
lower jaws, but not how much
wider (Cell, doi.org/dbqk).
“This doesn’t give us any idea
of what individuals from the Altai
looked like,” says Sheela Athreya at
Texas A&M University. “It’s based
on so very many assumptions
that it made my head spin.”The team validated the
method by using it to correctly
predict some known
characteristics of Neanderthal
bones. But Charles Roseman
at the University of Illinois at
Urbana-Champaign says the
method missed most Neanderthal
differences, meaning at best it
paints a very partial picture. ❚“This doesn’t give us any
idea of what Denisovans
looked like. It’s based on
so many assumptions”Human evolutionMichael Le PageNews
Marine biologyDiet may explain
why whales have
such big brainsTHE largest brains ever to have
evolved belong to whales. Now
we have discovered that the marine
mammals may have gained their
big brain size in the same way
we did: through massive expansion
of two particular brain regions,
fuelled perhaps by changes in diet.
Amandine Muller at the
University of Cambridge and
Stephen Montgomery at the
University of Bristol, UK, looked
at brain size data for 18 species
of whale and dolphin, as well as
for 124 different land animals,
including 43 species of primate.
With few exceptions, the whales,dolphins and primates all seem
to have gained large brains
through dramatic growth of
the same two brain regions:
the cerebellum and neocortex.
Both regions are important
for cognitive functions suchas attention and controlling
the movement of the body.
It makes sense that the
cerebellum and neocortex evolve
in unison, says Montgomery,
because they are physically
connected by many brain pathways.What drove these two brain
regions to expand so dramatically
in whales and dolphins? Muller
and Montgomery found that the
whale and dolphin species with a
larger cerebellum and neocortex
typically have an unusually broad
diet, in terms of the variety of foods
they consume. This may have
enabled the evolution of larger
brains (Journal of Evolutionary
Biology, doi.org/dbqs).
It is unclear why this would be the
case, but Montgomery speculates
that a broad diet is more likely to
provide the energetic resources
needed to fuel brain expansion. ❚
Colin BarrasGrey whales and humans
seem to have evolved their
big brains in similar waysMAAYAN^ HARELFirst glimpse of what a Denisovan
could have looked like
Ancient
DNA was
used to
produce this
portrait of a
Denisovan
womanMARK
CARWARDINE/NAT
URE^ PL