The Times Australia
The Times World News

.

Fossil teeth reveal how brains developed in utero over millions of years of human evolution – new research

  • Written by Tesla Monson, Assistant Professor of Anthropology, Western Washington University
Fossil teeth reveal how brains developed in utero over millions of years of human evolution – new research

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[1]. 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[2] 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
Pregnancy and delivery come with a lot of risks for parent and baby. Jimy Lindner/EyeEm via Getty Images[3]

Gestation and the human brain

Pregnancy and gestation are important periods[4] – 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[5] 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[6] 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[7]. The evolution of our large brain[8] 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[9], 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[10] 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[11] 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 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[12], 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[13] 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 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[14]

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[15], a trained expert can tell countless things[16] 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.

References

  1. ^ paleoanthropologists like me (scholar.google.com)
  2. ^ By building a mathematical model (doi.org)
  3. ^ Jimy Lindner/EyeEm via Getty Images (www.gettyimages.com)
  4. ^ Pregnancy and gestation are important periods (doi.org)
  5. ^ cost a lot of energy (doi.org)
  6. ^ tight fit of the infant’s head and shoulders (doi.org)
  7. ^ come along with having a big human brain (doi.org)
  8. ^ evolution of our large brain (doi.org)
  9. ^ cooperative group work (doi.org)
  10. ^ Humans have the highest prenatal growth rate (doi.org)
  11. ^ more than a quarter of all human brain growth (carta.anthropogeny.org)
  12. ^ Tooth enamel is more than 95% inorganic (doi.org)
  13. ^ We found that the rate of prenatal growth (doi.org)
  14. ^ CC BY-ND (creativecommons.org)
  15. ^ Even with only a few teeth and some of the jaw (doi.org)
  16. ^ a trained expert can tell countless things (www.annualreviews.org)

Read more https://theconversation.com/fossil-teeth-reveal-how-brains-developed-in-utero-over-millions-of-years-of-human-evolution-new-research-193167

Times Magazine

Headless CMS in Digital Twins and 3D Product Experiences

Image by freepik As the metaverse becomes more advanced and accessible, it's clear that multiple sectors will use digital twins and 3D product experiences to visualize, connect, and streamline efforts better. A digital twin is a virtual replica of ...

The Decline of Hyper-Casual: How Mid-Core Mobile Games Took Over in 2025

In recent years, the mobile gaming landscape has undergone a significant transformation, with mid-core mobile games emerging as the dominant force in app stores by 2025. This shift is underpinned by changing user habits and evolving monetization tr...

Understanding ITIL 4 and PRINCE2 Project Management Synergy

Key Highlights ITIL 4 focuses on IT service management, emphasising continual improvement and value creation through modern digital transformation approaches. PRINCE2 project management supports systematic planning and execution of projects wit...

What AI Adoption Means for the Future of Workplace Risk Management

Image by freepik As industrial operations become more complex and fast-paced, the risks faced by workers and employers alike continue to grow. Traditional safety models—reliant on manual oversight, reactive investigations, and standardised checklist...

From Beach Bops to Alpine Anthems: Your Sonos Survival Guide for a Long Weekend Escape

Alright, fellow adventurers and relaxation enthusiasts! So, you've packed your bags, charged your devices, and mentally prepared for that glorious King's Birthday long weekend. But hold on, are you really ready? Because a true long weekend warrior kn...

Effective Commercial Pest Control Solutions for a Safer Workplace

Keeping a workplace clean, safe, and free from pests is essential for maintaining productivity, protecting employee health, and upholding a company's reputation. Pests pose health risks, can cause structural damage, and can lead to serious legal an...

The Times Features

The Role of Your GP in Creating a Chronic Disease Management Plan That Works

Living with a long-term condition, whether that is diabetes, asthma, arthritis or heart disease, means making hundreds of small decisions every day. You plan your diet against m...

Troubleshooting Flickering Lights: A Comprehensive Guide for Homeowners

Image by rawpixel.com on Freepik Effectively addressing flickering lights in your home is more than just a matter of convenience; it's a pivotal aspect of both home safety and en...

My shins hurt after running. Could it be shin splints?

If you’ve started running for the first time, started again after a break, or your workout is more intense, you might have felt it. A dull, nagging ache down your shins after...

Metal Roof Replacement Cost Per Square Metre in 2025: A Comprehensive Guide for Australian Homeowners

In recent years, the trend of installing metal roofs has surged across Australia. With their reputation for being both robust and visually appealing, it's easy to understand thei...

Why You’re Always Adjusting Your Bra — and What to Do Instead

Image by freepik It starts with a gentle tug, then a subtle shift, and before you know it, you're adjusting your bra again — in the middle of work, at dinner, even on the couch. I...

How to Tell If Your Eyes Are Working Harder Than They Should Be

Image by freepik Most of us take our vision for granted—until it starts to let us down. Whether it's squinting at your phone, rubbing your eyes at the end of the day, or feeling ...