Domestic animals’ cuteness and humans’ relatively flat
faces may be the work of a gene that controls some important developmental
cells, a study of lab-grown human cells suggests.
Some scientists are touting the finding as the
first real genetic evidence for two theories about domestication. One of those
ideas is that humans
domesticated themselves over many generations, by weeding out hotheads in
favor of the friendly and cooperative (SN:
7/6/17). As people supposedly selected among themselves for tameness
traits, other genetic changes occurred that resulted in humans, like other domesticated
animals, having a different appearance than their predecessors. Human faces are
smaller, flatter and have less prominent brow ridges than Neandertal faces did,
Domesticated animals look different from their
wild counterparts as well. Shorter snouts, curly tails, floppy ears and spotted
coats are all traits that tend to pop up in domesticated animals. But until
recently, no one had an explanation for this “domestication syndrome.”
Then in 2014, three scientists
proposed that as people selected animals for tameness, they also happened
to select for genetic changes that slightly hamper
movement of some developmentally important cells (SN: 7/14/14). These neural crest cells are present early in
embryonic development and migrate to different parts of the embryo where they
give rise to many tissues, including bones and cartilage in the face, smooth
muscles, adrenal glands, pigment cells and parts of the nervous system. The
researchers’ idea was that mild genetic changes might produce neural crest
cells that don’t move as well, leading to domestic animals’ cuddlier look.
Both of those big domestication ideas have been
just that, with not much hard evidence for or against either. Some studies have
suggested that differences in some genes implicated in neural crest cell
function might have been important in the domestication of cats
(SN: 11/10/14), horses
(SN: 4/27/17) and other animals (SN: 5/11/15). But none of those studies
explained how those genetic differences led to altered behaviors or looks
between wild and domesticated critters.
In the new study, researchers studied cells from
people with developmental disorders to learn what makes neural crest cells
tick. One gene, BAZ1B, is a neural
crest cell boss, the team found, controlling 40 percent of genes active in
those cells. Altering levels of BAZ1B’s
protein affects how
quickly neural crest cells move in lab tests,
the scientists report December 4 in Science
Genes under BAZ1B’s
direction are among those that changed both in animals during domestication and
in modern humans as they evolved, the researchers also found. Some variants of
those genes are found in nearly every modern human, but either weren’t found or
were not as prevalent in the DNA of their extinct Neandertal or Denisovan
cousins (SN: 9/19/19), the team says.
That all adds up to one thing: “We’re giving the
first proof of self-domestication in humans,” says neuroscientist Matteo
Zanella of the University of Milan.
But Zanella and his colleagues’
conclusion is a giant leap from their research on cells growing in laboratory
dishes, says Kenneth Kosik, a neuroscientist at the University of California,
Santa Barbara. “It’s a very seductive paper,” full of interesting ideas and
reams of data, he says. But tying human evolution, domestication and
development of facial features together based on the activity of one gene is an
overinterpretation, Kosik says. “Those kinds of jumps just don’t belong in a
The researchers made their discoveries
by studying cells taken from people with two developmental disorders, each
involving a big piece of DNA from chromosome 7 that contains 28 genes,
including BAZ1B. People with Williams-Beuren
syndrome are missing that piece of DNA from one copy of chromosome 7, leaving
them with only one copy of BAZ1B and
the other genes. People with the genetic disorder are characteristically
talkative, outgoing and not aggressive, and also tend to have especially round
faces with short noses, full cheeks and wide mouths with full lips.
One the other hand, people with what’s
known as 7q11.23 duplication syndrome have an extra copy of that same piece of
DNA, giving them three copies of BAZ1B
and the other genes. People with the duplicated DNA have the opposite symptoms:
They tend to be aggressive, sometimes have difficulty speaking and have
autism-like characteristics that affect their ability to socialize. Their facial
features are also exaggerated but different from Williams syndrome.
That combination of behavior extremes
and exaggerated facial features seems to indicate that tameness and physical
changes go hand in hand just as researchers have proposed for human
self-domestication and domestication syndrome in other animals.
was already known to affect neural crest cell function. So probing its actions
in cells from people with the syndromes seemed likely to reveal more about how
modern human faces evolved, says neuroscientist and coauthor Alessandro
Vitriolo, also of the University of Milan. The researchers reasoned that
variations in BAZ1B and its protein may
slightly impair their function or how much of the protein is produced, leading
to slower neural crest cell movement and the characteristics of domestication.
But first, the team needed to know whether altering amounts of the BAZ1B protein
had any effect on neural crest cells. So the researchers reprogrammed skin
cells from people with Williams or the duplication syndrome into stem cells.
The scientists then grew the stem cells into neural crest cells.
For comparison, the team also made neural
crest cells from people who develop normally and who have the usual two copies
of BAZ1B and other 27 genes Also included
were cells from a person with a mild form of Williams syndrome. That person was
missing many of the genes in the region, but still had two copies of BAZ1B.
The team also used genetic tricks to
reduce levels of the BAZ1B protein, to be sure that any effects were due to the
BAZ1B gene and not one of the other
nearby genes. When researchers reduced these protein levels in each of the
different types of cells, the neural crest cells moved slower. Other genes’
activities were also influenced by the dose of BAZ1B and its protein in cells, the researchers found.
Those results correlating the amount of
BAZ1B protein with cellular biology is exactly what would be expected if a
neural crest cell gene were responsible for domestication syndrome, says Adam
Wilkins, an evolutionary biologist and one of the authors of the 2014 paper. The
most convincing bit of evidence for him was the discovery that BAZ1B seems to affect the activity of
some genes that have changed in modern humans from the forms seen in
Neandertals and Denisovans.
Without that link, the data “would
really be just an interesting set of correlations,” says Wilkins, an
independent researcher in Berlin. The researchers “have provided some genetic
evidence for linking gene activity to paleoanthropological history.” Still, he admits
that he has some uneasiness about the study’s sweeping conclusions, though he
was not ready to articulate those doubts.
Other researchers are more enthusiastic.
“This is the strongest test yet of the human self-domestication hypothesis, and
seems to support the idea that humans, like many other animals, have evolved
due to selection for friendliness that also shaped other features like our
faces,” says Brian Hare, an evolutionary anthropologist at Duke