Meet your inner fish
Tiktaalik - Eduard Sola CC-BY-SA-3.0 via Wikimedia Commons
Haeckel’s theory that human embryos pass through stages of development that recapitulate our evolutionary history has subsequently been proved incorrect. But it’s undeniable that human embryos do look awfully similar to other species, at least in the early stages of life.
Haeckel thought that we went through a 'fish' stage in the womb because our embryos appear to have gills during early development. In fact, these are pharyngeal arches, which form the basis of structures like facial bones in mammals, but do indeed go on to become gills in fish.
Rather than re-working evolution in the womb, biologists now believe that these kinds of common structures in early embryos are evidence of our shared evolutionary origins. So, there is a relationship between embryology and evolution, but in a way that’s far more intricate than Haeckel could ever have known.
Back in the days of Haeckel and Darwin, nobody knew about DNA or genes. The exact mechanisms underpinning evolution were a mystery, as were the curious forces that shape an embryo as it grows in the womb.
Today, we know that many thousands of genes control embryonic development, but we’re still figuring out exactly how they work.
Evolutionary developmental biology, or evo-devo as it’s often known, is a relatively new scientific field that looks at how our genes control the journey from fertilised egg to fetus, and what it can tell us about the underlying processes of evolution.
Development in the womb depends on a complex network of regulatory genes. Some of them, known as Hox genes, control when and where in a developing embryo particular genes are switched on and off, ensuring that your body parts all end up in the right place.
Hox genes act together with other regulatory genes, creating a chain of command where the instructions become more specific with each gene further down the line.
Importantly, Hox genes don't have any instructions for making specific body parts. Like the architects of a building, these regulatory genes map out the plans for the body as a whole but don't they do any of the building themselves.
Instead, they simply say “Make a leg here” or “Arms here, please”, activating other genes that are needed to make the cells and tissues of the appropriate limbs.
Regulatory genes, particularly those high up in the chain of command, are extremely similar between species. In scientific terms, we would say that these genes are highly conserved, and it’s even possible to swap some of them between species and maintain the same body plan.
These similarities explain why groups of related animals have similar features - for example, the four limbs of mammals and other tetrapods, or the neatly arranged vertebrate backbone.
Unsurprisingly, variations in such important regulatory genes can have huge effects on the resulting organism. For example, particular faults in fruit fly Hox genes can result in a fly with a leg where it should have an antenna, or two sets of wings.
Mutations in Hox genes and other developmental genes may explain how evolution can sometimes seem to happen very quickly, for example, when species gain additional segments, legs, or wings in what seems like an unfeasibly short period of time (at least, in evolutionary terms).
What might surprise you is that when species lose traits through evolution, they can still have perfectly functional genes that build those body parts, but their regulatory genes have undergone changes that mean they simply don’t get switched on.
For example, chickens and other birds evolved from dinosaurs who had teeth. Chickens still have all the genes they need to make teeth, but they aren't switched on as a chick grows inside an egg. Mutations in regulatory genes can turn these long-forgotten genes back on, making – you guessed it - hen’s teeth.
Other examples of similar evolutionary throwbacks include whales with legs, horses with toes and even humans with tails.
So although we don’t know all the specifics about how the genes that are responsible for development have changed throughout evolution, we do know that all vertebrates (that’s animals with backbones) are essentially the same under the biological hood.
The genetic recipe may have been passed on and modified for hundreds of millions of years, but the basic instructions largely remain the same. In short, our evolutionary history is written in our genes, and it’s a history that we can trace right the way back to the very first vertebrates: fish.
We and all other vertebrates evolved from fish, which explains the embryonic resemblances observed by Haeckel all those years ago. But how did we go from swimming around in the sea to walking about on land? It’s a question that scientists have been trying to answer for decades.
The answer came in 2004 when American paleontologist Neil Shubin and his team discovered a strange fossilised skull sticking out of some ancient rocks on Ellesmere Island in northern Canada.
After careful excavation, they realised that they had found the fossilised skeletons of three ancient creatures, dating back around 375 million years. They named this new species 'Tiktaalik' after the local Inuit word for "large freshwater fish".
But rather than being a pure fish, equipped with fins and gills for swimming, or a land-lubbing tetrapod with limbs and feet and lungs, Tiktaalik is something in between. It does have scales and gills of a fish, but also a flat, crocodile-like head with eyes on top. It has purely tetrapod ribs and a neck.
But while Tiktaalik has legs and bent ankle joints that look for all the world like they belong to a tetrapod that’s used to walking on land, these limbs end in fins rather than toes, suggesting that they spent plenty of time squelching about in the mud.
As Shubin puts it, this curious mixture of characteristics makes Tiktaalik a ‘fishapod’ - our oldest known land-dwelling ancestors, and the putative missing link (or transitional form) between fish and land animals
In his book, Your Inner Fish, Shubin argues that this all adds up to making Tiktaalik one of our key ancestors. And even though it’s 375 million years old, Tiktaalik has all the bony structures that you need to make a human - shoulders, elbows, legs, a neck, wrists - suggesting it has the same basic development blueprint as we do.
This isn’t the only evidence of our ancestors we can see on our bodies. Pinch your thumb and first finger together and look at the inside of your wrist, if you can see a prominent tendon, congratulations, you’re an evolutionary throw-back (don’t worry - I’m one too).
This is the palmaris longus - a tendon that’s useful for lemurs and monkeys who use their forearms to move around in the trees, but it’s not much use for our modern ground-dwelling lifestyle.
As a result, there’s no selective pressure acting on the genes directing its development so it’s starting to be lost from the modern human population.
Around 10-15% of people don’t have this tendon with no ill-effects at all, which does make it a fun conversation starter at parties. You’re welcome.
We’re just starting to understand what our genes can tell us about our evolution, and who knows what secrets we will uncover in the future. Perhaps what we currently consider to be so-called junk DNA actually contains the full story of evolution that we’ve been searching for.
In my first book, Herding Hemingway’s Cats, I talk about how the genetic control switches scattered throughout this non-coding DNA are like evolution’s playground, with small changes to switches having big impacts on the shape, size and structure of various body parts in different species, even though the genes themselves don’t change.
One thing is for sure: even though we’ve left our fishy phase long behind us, we still carry the evolutionary legacy of our ancestors with us. From Tiktaalik to today, it’s time to embrace your inner fish.
References and further reading
