Like every other living thing on Earth - past, present, and indeed future - we’re products of evolution.
This might be hard for some to believe, but our evolutionary history has been dictated by the same biological, ecological, and geological processes that have shaped other forms of life for billions of years.
There’s simply no separating our story from that of the other animals, as well as the plants, fungi, bacteria, and archaea that share our planet; go back far enough - about 4.2 billion years - and we all share a common ancestor.
If evolution has been a constant ever since the origin of life 4.2 billion years ago, then why do some claim that, for humans at least, this fundamental process has ground to a halt?
This isn’t exactly a fringe theory. In fact, in an essay entitled The Spice of Life published in 2000, the esteemed evolutionary biologist and palaeontologist Stephen Jay Gould wrote, “There's been no biological change in humans in 40,000 or 50,000 years. Everything we call culture and civilization we’ve built with the same body and brain.”
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The idea behind this claim is that once humans achieved large enough brains and developed sophisticated cultures, cultural evolution supplanted biological evolution. Yes, humans continued evolving, but in a cultural sense rather than a biological one - at least according to Gould and other scholars.
However, many disagree with this claim, suggesting the opposite is in fact reality. In their 2009 book The 10,000 Year Explosion: How Civilization Accelerated Human Evolution, Gregory Cochran and Henry Harpending argued that, “Human evolution has accelerated in the past 10,000 years, rather than slowing or stopping, it is now happening about 100 times faster than its long-term average over the six million years of our existence.”
To understand this complex, nuanced argument, it’s important we go back to the basics and answer some fundamental questions.
What is evolution?
Put simply, evolution is a process by which the heritable characteristics of a population of living organisms change over successive generations. This process is primarily driven by random genetic mutations and natural selection, though it should not be confused with these closely related yet distinct processes.
It’s best to think of evolution as the engine of a car, and these other processes as the fuel that powers this engine.
There are four main patterns of evolution, each driven by a combination of random genetic mutations, natural selection, and other mechanisms such as genetic drift, gene flow, and migration. These are: divergent evolution, convergent evolution, parallel evolution, and coevolution.
IWhat is a species?
The term ‘species’ is something we use to group populations of organisms whose members share a long list of common characteristics.
These groupings can be based on a variety of different observations, from genetic similarities to morphological traits. However, the concept of a ‘species’ is ultimately an invention of humans - a way for us to make sense of the natural world, which doesn’t always fit neatly into our defined boundaries.
For example, you may have learnt at school that a species is a population of organisms who can reproduce with one another to produce fertile offspring. This is perhaps the most common and agreed upon definition of a ‘species’, yet it doesn’t hold up in many cases.
Take wolves (Canis lupus) and dogs (Canis familiaris), they’re considered different species, yet they hybridise and produce fertile offspring quite often. The same is true of polar bears (Ursus maritimus) and grizzly bears (Ursus arctos horribilis), bison (Bison bison) and domestic cows (Bos taurus), and even us, historically.
Through genomic sequencing and analysis of ancient DNA, scientists have worked out Homo sapiens interbred with Neanderthals (Homo neanderthalensis) for thousands of years. And they must have done this quite regularly, as modern human populations outside of Africa carry approximately 1-4% Neanderthal DNA.
The most extreme example of two different species interbreeding and producing living offspring is the case of the American paddlefish (Polyodon spathula) and the Russian sturgeon (Huso gueldenstaedtii).
These fishes not only belong to two entirely different families, they last shared a common ancestor more than 141 million years ago. If we were to put this in more understandable, mammalian terms, it’d be like a giraffe successfully interbreeding with a hippo…
In short, the term ‘species’ is quite arbitrary and there’s no single definition that biologists agree on.
How many human species have there been?
For the last 40,000 years we’ve lived alone - the last humans standing from a large and diverse group that, just 300,000 years ago, comprised nearly a dozen species of walking, talking great apes.
If we define ‘human’ as members of the genus Homo, then there are currently 16 recognised species. This is certainly a liberal estimate and includes several species that may be: (1) known from very few fossils, (2) of uncertain classification, or (3) invalid and better viewed as a synonym of a valid species.
Our most familiar cousins are Neanderthals, but there are several others we shared our planet with for several thousand years, including Homo erectus, Homo naledi, Homo floresiensis, and an enigmatic, so-far-unnamed group known as the Denisovans.
There were many other species of ‘human’ that lived before we arrived roughly 300,000 years ago too, from the first Homo, Homo habilis, to the species many think discovered how to harness fire, Homo heidelbergensis.
If we stretch the definition of ‘human’ a bit further and include all of the hominins that evolved after our last common ancestor with chimpanzees and bonobos (who lived roughly 6.5 million years ago), then there may have been as many as 30 different species!
It’s clear, given this rich evolutionary history, that our group has a propensity to form new species.
Have humans evolved since becoming humans?
Contrary to Gould’s and many others' claims, humans have not stopped evolving. Since our species (Homo sapiens) emerged 300,000 years ago, we’ve been through countless rounds of evolution. This hasn’t resulted in the creation of any new species, but it has changed us in countless ways - to the point where we, modern humans, are very different to the first Homo sapiens.
One area that has undergone significant change is our brains, especially in terms of their size. According to several studies, human brains have decreased in volume by roughly 10-15% since peaking 20,000 to 30,000 years ago. This contraction is believed to have started roughly 10,000 years during the rise of agriculture and associated civilizations. Once humans started living in large groups they could share knowledge, reducing the need for such large, energy-hungry brains.
A similar kind of shrinkage is evident in our mouths. As diets and technologies have evolved over thousands of years, our jaws and teeth have become smaller. This means many modern humans lack enough space for their wisdom teeth (third molars) - a situation that keeps dentists happy and those inflicted by toothache miserable.
Other examples of recent evolution include lactose tolerance, which coincided with the domestication of cattle roughly 10,000 years ago, genetic adaptations to low-oxygen environments amongst populations living in the Himalayas, blue eyes, a 6,000-year-old mutation in the OCA2 gene that causes reduced melanin in the iris, and sickle cell anemia, a genetic adaptation against malaria.
These examples disprove Gould’s original claim, supporting Cochran’s and Harpending’s argument that, “Human evolution has accelerated in the past 10,000 years.” However, Gould’s original claim does have some merit, recognising the importance of culture on modern human evolution.
The changes happening to our bodies are biological in nature, but whereas natural selection once drove them, they are now mainly driven by culture. Indeed, cultural evolution may have supplanted biological evolution in modern humans, thought it hasn’t replaced it, nor stopped it.
Ultimately, the process of evolution is constant and every living thing is subject to it. The only way to stop this fundamental process is through extinction - something you could say we’re well on our way towards…
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