To put it simply: yes, humans are descended from fish. Not cod, haddock, salmon, or any modern species, but rather a group of trailblazing fish that emerged in the Late Silurian, roughly 425 million years ago.
This group of fish are known as the sarcopterygians and they were once incredibly abundant and diverse, including both small, invertebrate-eating species and large, predatory species that had a particular taste for other fish.
Now, sarcopterygians make up a tiny fraction of today’s diversity of fishes and are represented by just two species of coelacanths and six species of lungfishes.
Both extinct and extant sarcopterygians look like fish. They are, by all definitions, fish; but they have several traits that we don’t typically associate with fish. Chief amongst those traits are four fleshy fins that not only resemble primitive limbs, but are primitive limbs. Some sarcopterygians also possess a series of small, internal air sacs – the precursors to our lungs.
Around 375 million years ago, some now-extinct sarcopterygians used their primitive limbs to drag themselves out of water and onto land, gulping atmospheric oxygen as they went.
In doing so, these once water-bound pioneers changed the course of evolutionary history and spawned a dynasty that would go on to give rise to all of the land-dwelling vertebrates that have ever lived, from amphibians and reptiles, to birds and mammals.
What was the first fish to walk on land?
It’s unclear exactly what fish was the first to walk on land, but it’s widely agreed that, by 375 million years ago, a group of sarcopterygians known as the tetrapodomorphs had started to experiment with a somewhat semi-aquatic lifestyle.
For years, researchers searched for a transitional animal that would support their hypothesis that four-legged land animals evolved from fish. In 2004, a small team working in Arctic Canada and led by palaeontologist Neil Shubin discovered Tiktaalik.
This 2.5m-long, half-fish, half-tetrapod had two fleshy but robust pectoral fins, strong enough to prop itself up in shallow water and perhaps even crawl on land. This was the ‘fishapod’ that had long been hypothesised, the ‘missing link’ between fish and terrestrial vertebrates.
In life, Tiktaalik would have looked a lot like today’s crocodiles, with a flat, pointed snout, upward-pointing eyes, paddle-shaped limbs, and an elongated, streamlined body.
These adaptations would have made Tiktaalik a formidable predator in the shallow streams and swamps that it lived in. Based on these adaptations and other finds from Arctic Canada, it’s thought Tiktaalik likely hunted fish that shared its habitat, lying in wait before lunging at any that dared to enter its shallow domain. There are some that think it may have also hunted invertebrates that lived along the water’s edge, such as spiders, scorpions, and insects.
Tiktaalik lived 375 million years ago during the Late Devonian – a time when invertebrates were the most dominant form of animal life on land. Not long after Tiktaalik, though, the scales tipped and vertebrates rose to dominance, establishing many of the terrestrial ecosystems that we recognise today.
While Tiktaalik may not be a direct ancestor of ours, it was likely a close cousin of the so-far-undiscovered ‘fishapod’ that ultimately gave rise to amphibians, reptiles, mammals, and, eventually, us. This ‘fishapod’ probably looked just like Tiktaalik and, like Tiktaalik, had the fleshy fins it needed to experiment with life on land.
What came before Tiktaalik?
Our evolutionary story didn’t just suddenly start 375 million years ago with Tiktaalik, nor 425 million years ago with the first sarcopterygians. Like all vertebrates living today, we can trace our roots a lot further back through time, to a time before fish, even.
The sarcopterygians, the group that gave rise to tetrapodomorphs like Tiktaalik, belong to an overarching group known as the osteichthyans, or bony fish. This group, in turn, belongs to another overarching group known as the gnathostomes, or jawed vertebrates.
Today, almost all vertebrates are jawed vertebrates (save for hagfish and lamprey), but during the Silurian (~430 million years ago), most vertebrates were jawless and belonged to an enigmatic group of fish known as the agnathans. The agnathans have been around since the Cambrian (~518 million years ago) and likely evolved from a group of fish-like, sausage-shaped animals that lived a few million years beforehand.
These superficially ‘simple’ animals were amongst the first chordates who, themselves, were descended from the deuterostomes – a group of animals united by the fact that they’re bilaterally symmetrical and that their anus forms prior to their mouth during embryonic development. We belong to this early diverging group, as do all other living and extinct vertebrates.
The animals that lived before the deuterostomes were very… weird. These strange animals lived during the Ediacaran, more than 541 million years ago, and they were predominantly soft-bodied. To the untrained eye, these animals look like nothing more than squishy blobs of slime.
What was the first animal?
There’s a lot of debate surrounding the affinities of these strange, primitive animals that lived during the Ediacaran, as well as which deserves the prestigious title of ‘first animal’. After many years of intense study, researchers have whittled down the list of potential candidates to two groups: comb jellies and sponges.
Contrary to their name, comb jellies aren’t jellyfish. Instead, they belong to a group known as the ctenophores that, according to molecular clock models, emerged between 600 and 700 million years ago. These jellyfish-like animals are made up of a central mass of jelly and a number of hair-like projections, known as cilia, that they use to swim.
Sponges, on the other hand, can’t swim. They attach themselves to the seabed and use their tube-shaped bodies like a pump to circulate water and filter out microscopic foodstuffs. There are fossils of primitive sponges dated to 800 million years ago, and some that may be even older. However, the interpretation that these fossils are sponges isn’t so widely agreed upon.
Whatever the first animal was, it was no doubt small and very, very simple. From these humble beginnings, though, a diverse array of animals emerged, animals that are far from simple.
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Main image: a mudskipper in Daintree National Park, Queensland, Australia. Credit: Getty