Eve, Adam and how we almost didn’t make it
We’re incredibly lucky. The world could have been a completely different place; a world without humans. There would have been no Giza pyramids, no wall of China, no Roman empire, no International Space Station. *dramatic pause* No hummus.
It’s a story of hardship, disasters and conflicting theories.
In a world far far away, a long time ago
Modern humans show a very limited genetic variation. In fact, if we randomly take two people from anywhere in the world and compare their DNA, they would be more genetically similar than two mountain gorillas from the same troop*. All humans alive today are, in essence, cousins.** But how can that be? What happened to our genetic variation? To get the answer to that we need to – once again – visit those prehistoric grassy plains in Africa from where we came.
Several hundred thousands years ago, modern humans existed only in Africa, and were slowly expanding north through what was then the fertile grassy plains of Sahara. But then, some 70,000 years ago, the Indonesian volcano Toba erupted in a vast cloud of ash and smoke, triggering a volcanic winter of several years and kick-starting the latest ice age. Our population suddenly collapsed to a fraction of its previous size and only a few thousand humans remained in the whole world.
This was obviously disastrous. Our genetic gene pool shrunk to a puddle and even a single epidemic outbreak could easily had killed off all humans in one go. And thus, even to this day, human beings have a very limited genetic variation.
It’s a lovely theory, full of drama and perseverance against all odds. But unfortunately it looks like it might be incorrect. The Toba super-eruption did take place, and the latest ice age (or rather the latest glacial period in the current ice age) did start about that time, but studies of the full genetic material of humans show no evidence of a drastic population bottleneck at that time. Bummer.
There is however another theory that can explain our lack of genetic variation.
Going out with a fizz, not a bang
In the year 2000, a study of human population bottlenecks found that there had indeed been a drastic reduction of the human population. But it hadn’t happened 70,000 years ago. And it wasn’t a single dramatic event. Instead, the study found that early humans most likely suffered a sustained drawn-out population bottleneck effect ca. 2 million years ago. It seems likely that our world-wide population was as low as 2,000 individuals for perhaps as long as 100,000 years.
Now, 2 million years ago human beings weren’t modern. In fact, even though they were our direct ancestors and begun the unbroken lineage to the current human population, they were sufficiently different from us to be defined as a different species: Homo ergaster.
But regardless what chronospecies we belonged to, it seemed like we had a really hard time. With a world population of only 2,000 individuals for an extended period of time, we would – in todays conservation terminology – be classified as a critically endangered species. But, as luck would have it, instead of going extinct, we spawned several new species from isolated pockets of populations. And even if most of those species went extinct, more than a million years later Homo ergaster was still around, together with a sister species that had spread to Asia: Homo erectus. And Homo heidelbergensis had appeared (and would later give rise to Homo neanderthalensis, the elusive and still un-named Denisovans as well as modern humans: Homo sapiens). So our toughest challenge was also the key to our most proliferate speciation and helped us to spread across the world.
But a story about the origin of modern humans wouldn’t be complete without mentioning Adam and Eve. No, not the biblical ones. The real ones. Let’s start with Eve, since she came first.
All humans in the world today are descendants of a single woman who lived some 200,000 years ago. She is known as Eve, or Mitochondrial Eve.
Now, the concept of Mitochondrial Eve might warrant some further explanation. Mitochondria are the power plants of our cells, converting glucose and other molecules to ATP – the main chemical energy form for the cell. They have their own DNA, separate from the main DNA that is housed in the cell’s nucleus. It stays unaffected by cell division and recombination and only really change through the slow process of mutation. As mitochondria are inherited exclusively from the mother (sperm cells only transfer nuclear DNA to the egg cell during fertilisation), we can trace the human lineage on the maternal side by studying mutations in the mitochondrial DNA.
Even though all living human beings are descendants of a single female, it doesn’t follow that there was only one woman left, and that her children had to mate with each other (ew!). Rather, many thousand women probably existed, spread over a number of tribes. But chance would have it that all the other women’s daughters sooner or later didn’t have any daughters of their own. And so their lineages of mitochondrial DNA were broken.
The end result is that all humans alive today have inherited their mitochondria from the set that an ordinary tribes woman somewhere in eastern Africa carried in her cells some 200,000 years ago. Which I think is pretty cool.
There is also a real Adam, called Y-chromosomal Adam, even though he’s not as interesting as Eve.
Y-chromosomal Adam is the man whose Y-chromosome can be traced to every living man today. Since the Y-chromosome doesn’t recombine with other Y-chromosomes (only males carry one, and they only carry a single one), it can be tracked back in time like the mitochondria of Eve. But, since only men carry Y-chromosomes, only men are direct descendants of Adam, not all humans. Which is why it’s not as interesting as the mitochondrial Eve thing.
A bit more interesting is that Eve lived about 200,000 years ago and Adam some 142,000 years ago. That’s a gap of 60,000 years – give or take a few thousand years. So Adam and Eve never met. In fact, the biblical story sort of sounds like a crude misinterpretation of our phylogenetic history, as if it had been told to some illiterate goat herders many thousand years ago, passed on as a magic fairytale from generation to generation until finally written down thousands of years later. Which of course it couldn’t have been. I mean, who would have told them the story for a start? The human technology at the time – although impressive – did not include advanced genetic sequencing and supercomputers for analysing the results.
So it looks like the biblical Adam and Eve have very little to do with reality. Something we probably should be thankful for, considering the story’s inherent sexist, demeaning and generally disturbing nature. It is a fairytale, and not even a very good one.
Luckily the truth is much more interesting.
* Mountain gorillas live in groups of 20-50 animals called troops. One dominant male (a silver back) mates with all the females in the troop, limiting the genetic variation within the group. But even so, they display a much higher level of genetic variation than humans.
** Our limited genetic variation might in part help explain our instinctive dislike of incest. Since we’re all so very closely related already, any incestuous behaviour might result in heavily inbred offspring, with a high probability of them ending up suffering from some horrible inherited disease. This is not so much an issue in other, more genetically diverse, species. Like rats, for instance, who are much more liberal in their mating behaviour.