[The rate of prenatal growth directly relates to how big an adult
brain grows. How and when Homo sapiens‘ high prenatal growth rate
evolved has been a mystery, until now.]
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SUNDAY SCIENCE: FOSSIL TEETH REVEAL HOW BRAINS DEVELOPED IN UTERO
OVER MILLIONS OF YEARS OF HUMAN EVOLUTION  
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Tesla Monson
January 25, 2023
The Conversation
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_ The rate of prenatal growth directly relates to how big an adult
brain grows. How and when Homo sapiens‘ high prenatal growth rate
evolved has been a mystery, until now. _

Any hominid fossil find with molar teeth can be plugged into a new
equation that reveals its species’ prenatal growth rate., Gil
Cohen-Magen/AFP via Getty Images

 

Fossilized bones help tell the story of what human beings and our
predecessors were doing hundreds of thousands of years ago. But how
can you learn about important parts of our ancestors’ life cycle –
like pregnancy or gestation – that leave no obvious trace in the
fossil record?

The large brains, relative to overall body size, that are a defining
characteristic of our species make pregnancy and gestation
particularly interesting to paleoanthropologists like me
[[link removed]].
_Homo sapiens’_ big skulls contribute to our difficult labor and
delivery. But the big brains inside are what let our species really
take off.

My colleagues and I especially wanted to know how fast our
ancestors’ brains grew before birth. Was it comparable to fetal
brain growth today? Investigating when prenatal growth and pregnancy
became humanlike can help reveal when and how our ancestors’ brains
became more like ours than like our ape relatives’.

To investigate the evolution of prenatal growth rates, we focused on
the in-utero development of teeth – which do fossilize. By building
a mathematical model [[link removed]] using
the relative lengths of molar teeth, we were able to track
evolutionary changes in prenatal growth rates in the fossil record.
Based on our model, it looks as if pregnancy and prenatal growth
became more humanlike than chimplike almost 1 million years ago.

[pregnant woman's silhouette against sunset on landscape]
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Pregnancy and delivery come with a lot of risks for parent and baby.
Jimy Lindner/EyeEm via Getty Images
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Gestation and the human brain

Pregnancy and gestation are important periods
[[link removed](SICI)1520-6505(1998)6:2%3C54::AID-EVAN3%3E3.0.CO;2-W]
– they guide future growth and development and set the biological
course for life.

But human pregnancy, and particularly labor and delivery, cost a lot
of energy [[link removed]] and
are often dangerous. The large fetal brain requires a lot of nutrients
during development. The rate of embryonic growth during gestation,
also known as the prenatal growth rate, exacts a metabolic and
physiological toll on the gestating parent. And the tight fit of the
infant’s head and shoulders
[[link removed]] through the pelvic canal
during delivery can lead to death, for both the mother and child.

As a trade-off to those potential downsides, there must be a really
good reason to have such large heads. The justification is all the
abilities that come along with having a big human brain
[[link removed]]. The evolution of our
large brain [[link removed]] contributed to
our species’ dominance and is associated with increased use of
technology and tools, creation of art and the ability to survive in
diverse landscapes, among other advances.

The timing and sequence of events that led to the evolution of our
large brains is entangled with the ability to find and process more
resources, through the use of tools and cooperative group work
[[link removed]], for example.

By investigating changes in prenatal growth, we are also investigating
changes in how parents gathered food resources and distributed them to
their offspring. These increasing resources would have also helped
drive the evolution of an even bigger brain. Understanding more about
when prenatal growth and pregnancy became humanlike at the same time
reveals information about when and how our brains did too.

Humans have the highest prenatal growth rate
[[link removed]] of all primates living
today, at 0.41 ounces/day (11.58 grams/day). Gorillas, for example,
have a much larger adult body size than humans, but their prenatal
growth rate is only 0.29 ounces/day (8.16 grams/day). Because more
than a quarter of all human brain growth
[[link removed]]
is completed during gestation, the rate of prenatal growth directly
relates to how big an adult brain grows. How and when _Homo
sapiens_‘ high prenatal growth rate evolved has been a mystery,
until now.

What teeth can tell about prenatal growth

Researchers have spent centuries investigating variation in fossilized
skeletal remains. Unfortunately brains – let alone gestation and
prenatal growth rate – don’t fossilize.

[ultrasound of a baby in utero]
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The developing brain of a human being gestating at 26 weeks. Tesla
Monson

But my colleagues and I started thinking about how teeth develop very,
very early in utero. Your permanent adult teeth started developing
long before you were born, when you were just a 20-week-old fetus.
Tooth enamel is more than 95% inorganic
[[link removed]], and the vast majority of
everything we see in the vertebrate fossil record is teeth, or has
teeth.

Building off this realization, we decided to investigate the
relationship between prenatal growth rate, brain size and the lengths
of teeth.

We measured the teeth of 608 recently living primates from skeletal
collections all around the world. We compared those measurements to
rates of prenatal growth that we calculated from average gestation
length and mass at birth for each species. We also looked at
endocranial volume – essentially how much space is inside the skull
– as a proxy for brain size.

We found that the rate of prenatal growth
[[link removed]] is significantly correlated
with both adult brain size and relative tooth lengths, across apes and
monkeys.

Because prenatal growth is so tightly correlated with relative molar
lengths, we were able to use this statistical relationship to generate
a mathematical equation that predicts prenatal growth rate from teeth
alone. With this equation, we can take a few molar teeth from an
extinct fossil species and reconstruct exactly how fast their
offspring grew during gestation.

[alt]
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Using the new equation, researchers found that prenatal growth rates
increased over millions of years of human and hominid evolution. Tesla
Monson, CC BY-ND [[link removed]]

Using our new method, we then reconstructed prenatal growth rates for
13 fossil species, building a timeline of changes over the past 6
million years of human and hominid evolution. “Hominid” describes
all the species on the human side of the family tree after the split
about 6 million to 8 million years ago from the common ancestor we
shared with chimpanzees. From our new research, we now know that
prenatal growth rates increased throughout hominid evolution, reaching
a humanlike rate that exceeds what we see in all other apes less than
1 million years ago.

A fully human prenatal growth rate appeared with the evolution of our
species _Homo sapiens_ only around 200,000 years ago. But other
hominid species living in the past 200,000 years, such as
Neanderthals, also had “human” prenatal growth rates. Which genes
were involved in these changes in growth rate remains to be
investigated.

Equation means teeth now reveal even more

Even with only a few teeth and some of the jaw
[[link removed](200103)114:3%3C192::AID-AJPA1020%3E3.0.CO;2-Q],
a trained expert can tell countless things
[[link removed]] about an
extinct individual – what species it was, what kind of diet it ate,
whether it competed for mates through fighting, how old it was when it
died, whether or not it had any serious health issues and more.

Now, for the first time, we can add to that list knowing what
pregnancy and gestation were like for that individual and other
members of its species. Teeth can even indirectly hint at the
emergence of human consciousness, via evolving brain size.

Interestingly, our model suggests that prenatal growth rates started
increasing well before the emergence of our _Homo sapiens_ species. We
can hypothesize that having a fast prenatal growth rate was necessary
for growing that big brain and evolving human consciousness and
cognitive abilities.

These are the sorts of big-picture questions this research lets us
start to formulate now – all from just a few teeth.[The
Conversation]

Tesla Monson
[[link removed]], Assistant
Professor of Anthropology, _Western Washington University
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This article is republished from The Conversation
[[link removed]] under a Creative Commons license. Read
the original article
[[link removed]].

* Science
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* Evolution
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* Brain
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* Teeth
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* fossils
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* humans
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* pregnancy
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* paleoanthropology
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