Origins of Humankind, Part III: Why Humans?
Things are about to get personal... In episode three of the series, Chris Stringer guides us through the story of humanity itself.
The Origins of Humankind continues! Our guide this week is a true icon in the field: Chris Stringer from London’s Natural History Museum.
Together, we trace the origins of the genus Homo, as well as the emergence and spread of Homo sapiens. While doing this, we meet many oddities, such as rhino hunting in Britain, but we also explore some of the biggest questions in human evolution, such as:
What is a human?
Why did we evolve big brains?
Is Homo sapiens truly unique — or just one human among many?
You can now listen to the episode wherever you get your shows, or you can keep reading for highlights.
Listen
Listen (1h6min): Spotify | Apple Podcasts | Other Players
This is part of the Origins of Humankind series produced with CARTA (UC San Diego). You can enjoy each episode as a stand-alone, or head here for the whole experience.
Highlights
Prefer reading to listening? Not a problem. Here’s a summary of our conversation in six parts.
1. Who are we?
“What is a human?” I asked.
“Well, it depends,” Stringer responded with a laugh. On a narrow definition, only Homo sapiens deserves the term. But Stringer disagreed — if only because it raises awkward questions about the sex that did happen between Homo sapiens and other lineages, like the Neanderthals
“What would we call them if not humans? Animals?“
For Stringer, all Homo species deserve to be called human. So what earns an ape admission into this prestigious genus? Stringer’s list went as follows: We are apes with big brains who live fully on the ground, who walk upright, who rely on manufactured stone tools, and who have small teeth — especially small “canines”.
In comparison, “proto-humans”, like austrolopithecins, were upright walkers — just like humans — but they still had many adaptations for living in trees. Their canines were smaller than chimpanzees, but larger than those of humans. Their brains were barely bigger than the chimpanzees. And while they used and even made stone tools, they likely didn’t rely on them to the same extent as early Homo species.
“When I was just beginning my career, “Man the tool Maker” was the view; that only humans were toolmakers and tool users. We know that's not true. But it's the the extent to which we're really dependent on tools for survival, more than any other animal. So it's an essential part of the human adaptation.“
Armed with this evolving self-image, we turned to the fundamental question of human evolution: Why did we evolve?
2. Leaving the trees
Let’s start with ground living.
Why did our ancestors “leave the trees”? Stringer nodded in agreement when I explained the main theory discussed on the show time and time again: North and South America were joining around the time of human origins. As the Isthmus of Panama blocked warm ocean currents from the Pacific, Africa’s climate grew colder and drier. Forest cover retreated. A new ecosystem of grasslands and sparse woodlands was developing. With the slow but steady rhythm of evolution, humanity marched into the new niche.
This overview raises many questions — for example, why our ancestors stood upright while modern savanna-dwelling chimpanzees do just fine on all fours. But details aside, the picture seems relatively clear: the African climate was shifting, and with it came an opportunity for a terrestrial ape. Humans picked up the bill.
So that’s terrestrial living. What about the big brains?
The remarkable growth of the human brain hides two puzzles.
3. Fuelling the Brain
The puzzle first has to do with energy. According to a now-famous factoid, the brain is only 2% of our body mass but eats 20% of our calories. Dean Falk warned me against taking these numbers at face value. But whether they are exact or not, everyone agrees that our brains are a hungry organ. Indeed, the right question in brain evolution isn’t “why to have a big brain”. It’s rather, “how to afford a big brain”.
So how did we?
A famous answer argues that we did it by reducing the size of our gut. The gut is energy-hungry, too. According to the “expensive tissue hypothesis”, humanity swapped a small gut for a big brain. Indeed, even early humans had very slim bellies compared to australopithecine and chimpanzees.
Like all theories, this “expensive tissue hypothesis“ has its opponents. But details aside, we do have very small bellies. And this underlines the first of the two puzzles: How could early humans digest all the extra energy their brain needed while cutting the size of their digestive track? Did they cook their food with fire, as suggested by Richard Wrangham? Did they thrive on animal fats, like bone marrow or marine animals, which are easier to digest raw? Or did they ferment their foods by accident? What, if anything, can we say about this all-important question in human evolution?
Stringer said he finds Wrangham’s theorising about fire “very interesting” but doesn’t see evidence of fire use very early on. To make matters worse, he has excavated a Neanderthal site where meat was eaten raw despite the presence of fireplaces. By whatever means, these Neanderthals could fuel their massive brains — bigger than ours — without frying the steak.
Stringer took a similar view on aquatic foods: they were probably useful for some populations but not all. He didn’t comment on the fermentation theory. Instead, he added two important extra points, both linking to the importance of stone tools in human evolution.
First, “cooking” isn’t the same as “cooking with fire”.
“It doesn't necessarily have to be heated: it can even just be pounded and that makes it easier to digest as well to unlock the main source in nuts and so on. You break them open, you crush them down, they're easier to digest.”
Furthermore, any discussion on meat is bound to invite ideas of palaeolithic “steaks”. (See my earlier point!) But this is a mistake. Humans didn’t just eat muscle. They ate intestines. More precisely, humans would almost certainly eat the guts of other animals, which apparently “taste a bit like sour yogurt”, Stringer told me.
Crucially, the gut would provide humans a nutritionally elevated experience of the diet of the prey animals. The reindeer’s gut would break down berries. Humans would swallow the product. Shrinking our own guts couldn’t get more mechanical than this.
(On a side note, Stringer’s anecdotal evidence for “meat beyond the steak” comes from a truly extraordinary site in Boxgrove, England. The whole site is the stuff of tales: pre-Sapiens humans lived here half a million years ago, alongside hippos swimming in the River Thames. The “meat” on the menu was rhino brain.)
So that’s on fuelling the brain. But building a big brain takes more than fuel. It takes time. This brings us to the second puzzle: the puzzle of childhoods.
4. Slowing Down, Speeding Up
Many tearful parents have commented that time flies when children grow. Suddenly, they are small adults leaving home. But biologically speaking, the opposite is true: humans really take their time growing up.
Tim Coulson explained in part I of this series that all primates take life slowly. But humans have taken this pattern to an extreme. And it makes sense: brains take time to grow. But here’s the puzzle: in most animals, a slower life history means longer birth intervals. Yet humans have extremely short birth intervals. Chimpanzee siblings are never less than seven years apart. Amongst humans, three years is typical, and shorter is doable. So how could humans have both slower life histories and faster birth intervals? How could mothers afford this?
The short answer is simple: mothers couldn’t afford it. Not alone. Enter human-style child-rearing — the kind that truly “takes a village.” (It wouldn’t hurt to have a pair-bonded dad sticking around, too.)
Stringer agreed with this basic idea while acknowledging that this is not his expertise. He also added that childhood mortality was very high amongst pre-modern humans, even among modern hunter-gatherers who otherwise lived relatively healthy lives. So, the human pattern might have exchanged some quality for quantity. But even then, the puzzle remains: why go towards quality on the length of childhood whilst picking quantity on the length of birth intervals?
The means remain a mystery, but humanity did stumble onwards. Our brains grew. Our childoods got longer. Our teeth got even smaller. But this was not a simple story leading from Homo erectus to Homo sapiens. Before our domination, the Earth was full of different human species, each different from the other. So who were they? Which of them were our cousins? And which of them gave rise to us?
5. Muddle in the Middle Earth
Stringer is best known for his pioneering work towards the “Out of Africa" theory of human evolution. In his PhD work, he travelled across Europe to measure ancient skulls. As he recalls in his 2012 book, Lone Survivors:
“I survived many adventures, including several border confrontations and two robberies, but by the end of my 5,000-mile trip I had collected one of the largest data sets of Neanderthal and early modern skull measurements assembled by anyone up to that time.”
At the time, many believed that Neanderthals were the direct ancestors of modern Europeans. This was part of a broader “multi-regional” theory of human evolution, where modern “races” had deep roots in the distant past, stretching back over a million years. In this model, Europeans descended from Neanderthals, Chinese from the Asian Homo erectus, and so on, with small genetic flows ensuring that these “races” stay able to reproduce with each other.
Measuring Neanderthal skulls, Stringer noticed that this couldn’t be the case. As Stringer wrote:
“Neanderthal skulls were no more similar to those of recent Europeans than they were to Africans, Eskimo, or Native Tasmanians, and Cro-Magnon skulls did not neatly slot between the Neanderthals and recent Europeans. Early modern skulls from around the world seemed to cluster with their modern counterparts rather than with any archaic skulls from the same regions. ”
It took some time to work out the details, but “The Out of Africa” model grew directly out of Stringer’s PhD thesis: All Sapiens populations come from a recent ancestor in Africa. In this model, Neanderthals were not our ancestors but our cousins.
And Stringer was right — with a small caveat. Genetics has discovered small bits of Neanderthal DNA in most non-African populations. I have some. This shows that some “cousin marriages” has complicated the evolutionary family tree. But more than anything, genetics has vindicated Stringer’s groundbreaking work: Small admixtures aside, we are all from Africa.
But that doesn’t mean the story has gotten any simpler. The sheer number of human species has been growing fast. In Asia, we’ve found evidence of Denisovans and the “Dragon Man” — two species that Stringer believes might be the same. On islands of Southeast Asia we’ve even found small Hobbit-like species, like Homo floresiensis and Homo luzonensis. In Southern Africa, we find another human of remarkably short stature: Homo naledi, who buried its dead despite having a chimp-sized brain. The famous Homo erectus was probably still alive in Asia during the dawn of Homo sapiens, 300 thousand years ago. At this time, the Earth had an uncanny resemblance to Tolkien’s Middle Earth.
So, which of these many species is our real ancestor? If Neanderthals are our cousins, who is our shared grandparent?
The typical answer is Homo heidelbergensis — the same species that was buthering rhinos in pre-historic Britain. Stringer has grown sceptical of this model, suggesting that the split between Neanderthals and humans probably happened earlier. But we don’t know for sure. The period between the rise of Homo erectus and the dawn of Homo sapiens — is often called the “Muddle in the Middle”. We don’t know much about what happened here. But on this front, Stringer was beaming with optimism:
“Ask me again in two years. I think we will sort out the muddle in the middle.”
Hopefully, we will one day have a clear family tree, linking us to our ancestors, like Homo erectus, and our cousins, like the Neanderthals. But whether we achieve it or not, one puzzle remains: Why are we the only branch left?
Will we sort out this Muddle in the Middle Earth? Subscribe to On Humans, and I’ll keep you posted!
6. We, The Lone Survivors
Why are we the only humans left? Many answers tend to gravitate towards violence. “Do you think Neanderthals gave up the game without a fight?” one social media user asked me.
Well. Violence might certainly have happened. To be frank, we don’t have any evidence against our ancestors butchering Neanderthals. But we don’t have any evidence for it either. And crucially, we have many alternatives. As Stringer pointed out, Neanderthals might have been on the way out long before Homo sapiens entered their territories.
“Neanderthals were a species at the verge of extinction.”
And having more competition about the same Ice Age game hardly helped them. Indeed, the demise of the Neanderthals might have been as simple as increasing famines due to harsh climate and a new compatetitor for the same sparse game.
Indeed, Stringer believes that our numbers gave us one clear advantage over Neanderthals: our societies were more complex.
“Neanderthals and those earlier humans were living in smaller groups. probably less well connected across the landscape. Now networking is important to spread information and knowledge. Let's imagine that 300,000 years ago in Germany, there are a few [people who know how to] make spears. Now if those individuals die in a hunting accident or disease, the group has lost that knowledge entirely. So larger groups and larger networking gives you that greater capability of maintaining your knowledge.”
Crucially, both archaeological and genetic evidence points to Homo sapiens having broader networks of knowledge and gene exchange.
“That was an advantage for Homo sapiens.”
I suggested that this might relate to one of the most noticeable curiosities in our fossils: the lack of a brow ridge.
Just compare the brow ridge below of three human species beliw:
All but Sapiens have some pretty hardcore brow ridges.
Here is a comparison of Sapiens to Neanderthals. You can probably guess which one is ours.
So what does this tell us? According to Brian Hare, this is part of a “domestication syndrome” — an ill-understood set of changes that occur when animals evolve more docile tendencies. Here is a relevant illustration, famous for its appearance in Rutger Bregman's best-selling Humankind.
Here is how Richard Wrangham — anohter proponent of the “self-domestication theory” — explained things to me:
“ If you are an archeologist and you find a canine: is it a dog, is it a wolf? The way you decide is: does it have the characteristics that dogs tend to have compared to wolves or that domesticated animals generally tend to have compared to their wild ancestors? .. And Homo sapiens show all those features in relationship to their earlier ancestors.”
According to Hare and Wranghma, Homo sapiens evolved by selection against impulsive or “reactive” aggression.1 Outcome? Less brawls, more cooperation. And here’s the link to Stringer: A more docile temperament might have been the biological condition for bigger groups and larger networks — the phenomena Stringer emphasised in Lone Survivors. And it explains this while tying into the most particular of our “hard” features: our small brow ridge.
So what does Stringer make of all this?
Stringer nodded in agreement to the self-domestication ideas but didn’t go as far as endorsing it directly. Instead, he told me that the mystery of our brow ridge is a big mystery indeed — so big, in fact, that one “experimental anthropologist” in the 70s wore an artificial band of erectus-style brow ridges for 6 months and reported the results. One result was clear: people were scared of him, “which might link to Wrangham’s idea about aggression”. But unlike Wrangham and Hare — who focus on brow ridges as an almost accidental signal of more interesting hormonal changes — Stringer was keen to find a direct adaptive story. He suggested that they could be part of an aggressive signalling system. But this doesn’t necessarily mean that Homo sapiens stopped signalling aggression.
“We have many other ways of signaling. When you think of when you see a person: they may have a different hairstyle, they may have tattoos, they may have piercings, They may have makeup — their body may be adorned with all kinds of extra things like necklaces and jewelry and things. Plus they might have a great big weapon as well — that would also be a powerful signal. So I think that we have left that behind — that big brow ridge — as a signaling system.”
Perhaps. Or perhaps Wrangham and Hare are right, and the explanation lies in hormones, which Stringer said “will definitely play a part, too”. In short, we don’t quite know.
But amongst the many remaining unknown, I wil try to learn from Stringer’s optimism about the way forward. During our covnersation, he was most curious about “proteomics” as a way to move into the early epochs from which DNA doesn’t survive. But more generally, he might be the most wonderfully forward-looking thinker I’ve interviewed. He has worked for over 50 years at London’s Natural History Museum — “childhood dream come true” — and shows no signs of losing enthusiasm. What was especially meaningful for me was a small hint he left about the possible ways that science journalism can change lives. Talking about his upbringing in the Cockney area of East London, he recalled how:
“…there was a school broadcast on BBC called “How Things Began” that we used to listen to. I was about eight or nine when they had a broadcast about the Neanderthals on this program. And so there was this crackly radio speaker in the classroom and we had a poster on the wall to go with these talks. And it was by the artist Morris Wilson who had painted a picture of Neanderthals burying a child. And in the program they reconstructed this with these wailing Neanderthals mourning their lost child. This gripped me.”
I couldn’t help mentioning my genuine wish that some clever young person out there hears our conversation today and is gripped in a similar way.
“I hope so,” Stringer replied, with great warmth in his voice.
Thanks for reading! This was episode III of The Origins of Humankind. Next week, we will zoom into the journey of Sapiens with more detail — and face a big plot twist in the story of Neanderthal extinctions. Subscribe for free and join the journey!
Want to listen to the episode? Here are the links again: Spotify | Apple Podcasts | Other Players
Finally, if you want more tales from post-war London, check out this clip from our conversations. It follows my asking how Stringer got into a career in anthropology. Trust me, you want to hear this!
Wrangham often highlights that we have low “reactive aggression” (think pub brawl) but high “proactive aggression” (think assasinations). He explained this to me with reference to Sarah Hrdy’s famous thought experiment: you cannot put a hundred chimpanzees on a plane and not expect a fight. With humans, the fight is an oddity. Wrangham agreed with Hrdy. But here’s his twist: humans have to have airport security due to our capacity for strategic (“proactive”) violence. This distinction lies at the heart of Wrangham’s goodness paradox, explored in his book of the same name.
As it happens this On Humans episode drops in my podcast queue alongside this Talking Indonesia episode about bio-archeology in Indonesia - mentioned a few times.
A good companion episode:
https://indonesiaatmelbourne.unimelb.edu.au/talking-indonesia-archaeology-and-reading-bones/