Snail hunting in Lake Tanganyika and the puzzle of deep time
I was listening to a fascinating podcast by Dr Ellinor Michel, an eminent senior ecologist at the London Natural History Museum. Her name rang a bell, then I remembered I’d met her some time back – in a bikini, upside down in Lake Tanganyika, sticking numbered snails back on rocks where she’d borrowed them from. Here’s the story.
“The life shape of a snail is an expanding circle turning through space and time. It’s simple geometry, and it’s so beautiful.” I gave Ellinor a hard look. That’s an impressive way to begin a discussion about gastropods.
Beyond her temporary office, Lake Tanganyika glistened turquoise in the bright morning sun, a fuss of little fishing dories speckling its surface. Out on the veranda a few research students were clustered around two buckets, scrubbing snails with some defeated toothbrushes.
Ellinor detected my quizzical look and chuckled: “When I was younger my heart wanted to be an artist but my head delighted in mathematics. So I ended up being an evolutionary biologist studying snails. It satisfied both tendencies.
“Using morphometrics, anatomy, genetics, ecology and biogeography, I’m testing hypotheses of speciation for Tanganyikan gastropods. It’s molecular systematics.” I nodded knowingly but didn’t understand a word.
I’d met Ellinor by accident – she was looking for oxygen in the same place I was enquiring after accommodation.
“Oxygen for what?” I’d asked.
“Snail diving,” she’d replied.
I thought she was having me on. But she wasn’t.
Her snailing ground, Lake Tanganyika, is a huge, ancient and mysterious body of water formed when tectonic movements began tearing Africa apart along the Great Rift Valley (which is widening at about the speed your fingernails grow).
After Lake Baikal in Russia, it’s the second deepest (1,470 metres), second oldest (9–12 million years) and the second most diverse lake in the world. It holds nearly one litre in every five of the world’s unfrozen fresh water and is so deep that below 200m its waters have no oxygen and no animal or plant life.
Tanganyika is thought to be the “mother lake”, providing the ancestors of fish, gastropods and other forms of life which colonised lakes Victoria and Malawi. At last count, it was home to 250 endemic species of fish and around 60 endemic snails – a biologist’s dream.
That explained Ellinor’s interest in its crystal-clear waters. She reckoned on another 30 snails soon to be described and wouldn’t hazard a guess at the number of cichlid fish yet unknown to science. She had a nose for big questions.
“Two generations ago my ancestors were missionaries,” she explained. “My father’s a philosopher. So I grew up among people who asked questions which go to the root of existence: the meaning of life. We were always allowed to be intrigued by concepts. I ask the same questions as they did, I guess. I’ve just changed the methodology.”
Evolutionary biologists, I was to discover, tend to ask jaw-dropping questions. Among Ellinor’s kind can be listed Charles Darwin, Alfred Russel Wallace, Jean-Baptiste de Lamarck, John Haldane, Julian Huxley and others: exciting people, when you can get your head around their conceptual timespans.
Gastropods, it seems, are an eminently respectable starting point for evolutionary inquiry. Ellinor picked up a tray of similar-looking snails lined up in orderly rows, largest to smallest. They seemed much the same to me, but it turned out they were all different species.
“I’m not so interested in the creature inside,” she commented, prodding them thoughtfully. “They’re basically slugs. But their shells are a window on deep time. They were there at the beginning of hard bodies – maybe 600-million years ago.
“Because they grow in a spiral, each shell is also a record of the creature’s entire lifespan. Not many organisms do that: most get rid of their juvenile shape.”
Small aquatic things at the edge of time have had a large impact on evolutionary biology. Sharks teeth come to mind, as do barnacles.
The teeth have to do with a man named Nicolaus Steno who, in 1677, became the Titular Bishop of Titiopolis (now part of Turkey). He was given some fossilised sharks’ teeth from a quarry while dissecting a shark’s head and found the teeth to be identical. This posed a problem: if God in his perfection had made the Earth in seven days, why did He place sharks’ teeth inside rocks? It also raised a more practical problem: how did solid bodies get inside other solid bodies?
Steno’s genius was to recognise that all the troubling objects of geology were solids within solids and an answer to how they got there might be a key to the unravelling of the Earth’s structure and history.
The sharks’ teeth, he reasoned, were inside the rock because they had solidified before the rock enclosed them. Therefore the rocks must have been created after God’s Creation; formed, rather, by deposits from rivers, lakes or oceans. And because fossils were often found on mountains, the processes that got them there spoke of great movements of the Earth’s crust and great gouts of time.
The Titular Bishop of Titiopolis is today, quite rightly, considered to be the father of modern geology.
Unlike the sharks’ teeth, which advanced natural science, barnacles were so fascinating they retarded evolutionary thought for eight years. Charles Darwin, with all the material on hand to arrive at his epoch-changing theory of natural selection, was side-tracked by these potentially boring little rivets. In hindsight he said it was maybe the silliest thing he ever did.
A barnacle hatches from its egg as a tiny, six-legged beastie with one eye. After a few moults it transforms itself into an animal with three eyes, two shells, six pairs of legs, and an inclination to give up the roving habits of its youth and settle down. It doesn’t eat: its sole task is to find a suitable neighbourhood in which to attach itself, whereupon it transforms itself into a barnacle which never again moves.
What has become known as “Darwin’s Delay” maybe wasn’t merely the fact that he was fascinated by barnacle reproduction (they have penises up to seven times their shell length), but that, being the reclusive homeboy he was, he identified with their tendency to hunker down and never move again.
However, prodded by an article sent to him by a young naturalist, Alfred Russel Wallace, which set out the principles of natural selection (and scared Darwin nearly out of his wits), the bearded Victorian got back on track and soon afterwards produced the path-breaking work Origin of the Species.
What interested Ellinor, like Darwin and the Titular Bishop, was deep time. The fact that at least 90% of the creatures that ever existed have become extinct suggests that evolution is not the execution of some perfect, divine plan. There is, today, no doubt that evolutionary processes are at work. But some niggling questions remain.
Why, for instance, have some creatures – such as sharks and frogs – hardly changed at all in millions of years while gastropods and cichlids in the Great Lakes have speciated like starbursts?
In Lake Tanganyika, there are even left-handed and right-handed scale-biting fish (their mouths are twisted to one or other side and they eat scales from living fish) and some cichlids which, it is speculated, live on only the eyes of their prey which they dash in and snatch. In almost every conceivable niche there is a creature specially adapted to fill it.
If survival of the fittest is the rule, why do less adapted species persist and seemingly thrive? If learning to fly provided birds with great new opportunities, why is the penguin very successfully turning itself back into a fish? If a cow developed such a complicated stomach for digesting grass, why is a horse, which eats the same stuff, so successful with a simple gut?
Is there really a gene which produces left-handed or right-handed scale biters, or shifts the eyes of a flat fish round to the same side after birth?
Can we honestly expect chance and time to evoke the multiple modifications necessary to make the human eye? Even Darwin commented that “the thought of it makes me shudder”.
Somewhere between the Creationists and the Neo-Darwinists maybe there is a “third way”, a Lamarckian tendency for forms to be what they are because of some built-in response almost amounting to self-design. Heretical? Well, so was the Titular Bishop of Titiopolis.
Back to the lowly gastropod.
In Lake Tanganyika, as elsewhere, they have developed thick shells because crabs delight in peeling and eating them. So, in a deadly “arms race”, crabs have developed larger, stronger pincers, which has prompted the development of more complex, knobbly snail shells which are even harder to crack. Well and good in evolutionary terms.
But does that explain the shape change of a hundred different species? Why are there so many forms – seemingly far more than necessary?
In order to understand evolution, biologists need to figure out how a particular body shape develops. There must be some “laws of form” but, unhappily, they don’t yet know what these are.
The little shell in Ellinor’s hand was a beautiful thing – truly an expanding circle turning through space and time. The puzzle it poses, though, far exceeds its size.
“I keep asking myself: What are the motivating causes of diversity?” she commented as she dropped the armoured slug back into a bucket of water. “How much randomness does life need?”
A few hours later I’d been roped into diving off a secluded beach south of Kigoma with handfuls of gastropods. Each one, duly numbered, we placed back on a rock to continue with its life. The water was startlingly clear and we could see the rehabilitated snails chewing away at the algae among thousands of their kind, quite unaware of their role in the meaning of life.
Ellinor, ever the teacher, spat out the snorkel and, with a twinkle in her eye, offered me an opinion entirely characteristic of an evolutionary biologist: “We don’t prove things in science. We only show them to be increasingly improbable.”
Then she dived back down into the mother lake with another handful of snails. DM/ ML
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