Science: Eureka!

UNDER THE MICROSCOPE; THE JOYS AND FEARS OF DISCOVERY

JUST EVERY now and again we scientists cry Eureka! when we think that we have solved some problem or understood some process that we have been puzzling over. Such a sudden illumination is not peculiar to scientists, and others in quite different fields have similar delightful experiences. But because the original crier was the amazing Archimedes, we rather regard it as ours alone. I recently had a Eureka! experience.

I have been thinking for some years about the origin of multicellular organisms and the egg during evolution. The original life forms were single cells and they were, some 700 million years ago, doing very well. They were both numerous and varied and reproduced happily by cell division. So why did multicellular organisms evolve - what was the advantage of being a collection of cells? The standard answer is that it provided the possibility for division of labour among the cells so that some could become specialised for particular tasks, such as digesting food. But this could only come much later for they had first to be multicellular.

I had already published one model in which a cell all those millions of years ago grew large without dividing, perhaps because there was an excess of nutrients about or perhaps because of a mutation. After it had become large it might then have gone through a series of divisions and the cells could have stayed together. The advantage might have been that it was now too large to be captured by other cells that shared the same environment. I did not much like it and nor did anyone else. And I was fully aware that such ideas were a bit like the Just So stories of Rudyard Kipling which told, for example, that the elephant got its trunk because a crocodile pulled its nose.

But I kept the problem turning over in that part of my mind where I keep such things, until a young zoologist told me about the way the egg of the sponge eats the cell that feeds it. Eureka!, well, a little one. Eating was the solution. How could I have missed it. The initial advantage of being multicellular was that in hard times the cells could eat their sisters and so survive.

Imagine then a scenario in which a cell undergoes a mutation so that when it divides into two the daughter cells remain together and, with further divisions, they continue to stay together and so form a multicellular colony. When the colony gets too big it fragments and the cells continue to grow. The selective advantage promoting the survival of our organism was that when food was in short supply and single-celled organisms were dying all around, some of the cells in the colony could eat their neighbours and so ensure survival. There are in fact current examples of this process in animals like hydra and flat worms that, when starved, maintain their smaller form by some cells eating others.

It is then not too difficult to imagine that certain cells near the centre of the primitive colony became specialised for eating their neighbours and began to do so even when times were not bad. These cells are the origin of the egg, for they could eat and grow large and then divide to give a number of cells that could form a colony.

In all animals one can think of the egg as the cell for which the other cells sacrifice themselves for it is the only cell that survives because it gives rise to another animal. So one of the major functions of all the cells of the body is to ensure that the egg is fed - indeed, in the fly there are special nurse cells that feed the growing egg. So our origins may lie in cannibalistic altruism or altruistic suicide. Will I ever know if I am right?