In the final part of our series on the origins of Africa’s wildlife, William Gray delves into the continent’s seas and rivers in search of animals that have stood the test of time. His conclusion? There’s something distinctly fishy about our past.
Technically speaking, the term ‘living fossil’ is an oxymoron – a coupling of words that seem to contradict each other. By definition a fossil is something long dead. But let’s not be pedantic about this. The fact is that back in 1938, fishermen off the east coast of South Africa peered into their nets and saw a strange fish flopping about that would unleash a wave of excitement through the scientific world.
The fish was a coelacanth, a creature that palaeontologists had first discovered (well and truly fossilised) in rocks nearly 400 million years old. Everyone thought it had died out 70 million years ago and yet there it was, flesh, blood and scales – a relic from the past. Fourteen years later a second coelacanth was caught – the first of numerous specimens snagged from depths of 200-300m around the Comoros Islands between Madagascar and Mozambique.
Then, in 2000, a group of divers in Sodwana Bay (on South Africa’s wild northeast coast) hit the jackpot. At a depth of 104m Pieter Venter and fellow divers spotted no less than six coelacanths. “This means there must be a large colony that was hiding from the world all this time,” said Venter. The team managed to obtain video footage of three coelacanths, ranging in length from one to two metres, feeding off the ledge of an underwater canyon.
You may be wondering what all the fuss is about. After all, a coelacanth is just a fish – and a pretty ugly one at that. With big, gelatinous eyes, blotchy scales and gaping maw it resembles a grouper that’s gone well past its sell-by date. But this is no ordinary fish. Not only has it survived relatively unchanged for several hundred million years, allowing us to look back in time, but it also provides living evidence of a critical stage in evolution.
Look closely at a coelacanth’s pectoral fins and you can’t help but notice their stocky, limb-like appearance. Mechanically, these so-called ‘lobe fins’ could provide support out of water. Now just imagine a primeval swamp containing ancestral coelacanths squirming in the shallows, tempted by edible insects on land and wary of predators in deeper water. This could have been that pivotal moment of life on earth when fish first ventured onto land and sowed the evolutionary seed of all terrestrial vertebrates. The coelacanth, dubbed ‘old four-legs’ because of its four lobed fins, is the last of an ancient line of fish that may well have given rise to the first amphibians. They, in turn, were the ancestors of the reptiles, birds and finally the mammals. Yes, you guessed it. Gaze on a coelacanth and you may, in a very distant way, be looking at your lobe-finned forebear.
But don’t get too sentimental because that’s not the end of the story. The ‘modern’ coelacanth, now confined to deep water, doesn’t hold any clues as to how its adventurous ancestors solved the problem of breathing out of water.
However, another living fossil, the lobe-finned, air-gulping African lungfish (whose fossil relatives have been found alongside those of coelacanths), proves that primitive fish were well on the way to overcoming this dilemma. But here’s the catch: neither the lungfish nor the coelacanth can be regarded as the direct ancestors of all backboned landlubbers. Their skull bones simply don’t conform to the fundamental pattern that’s evident in all amphibians.
So where does that leave us? Well, there was a third group of lobe-finned fish known as rhipidistians and one of them, the eusthenopteron, clearly anticipates the archetypal terrestrial tetrapod. Uncovered in Quebec from rocks some 380 million years old, the eusthenopteron has never shown up in any fisherman’s net. But then no one ever expected the coelacanth to reappear out of the blue. One day we might, just might, come face to face with our most direct fishy ancestor.