Multicellularity: Life’s greatest invention?

In Newscientist.com’s article on Life’s Top 10 Greatest Inventions, multicellularity emerged as the greatest invention. Read the excerpt below to understand how this came about.

Multicellularity

Volvocine Series

Ponder this one in the bath. Chances are you’ve just scrubbed your back with a choice example of one of evolution’s greatest inventions. Or at least, a good plastic copy.

Sponges are a key example of multicellular life, an innovation that transformed living things from solitary cells into fantastically complex bodies. It was such a great move, it evolved at least 16 different times. Animals, land plants, fungi and algae all joined in.

Cells have been joining forces for billions of years. Even bacteria can do it,forming complex colonies with a three-dimensional structure and some division of labour. But hundreds of millions of years ago, eukaryotes – more complexcells that package up their DNA in a nucleus – took things to a new level. Theyformed permanent colonies in which certain cells dedicated themselves to different tasks, such as nutrition or excretion, and whose behaviour was well coordinated.

Eukaryotes could make this leap because they had already evolved many of the necessary attributes for other purposes. Many single-celled eukaryotes can specialise or “differentiate” into cell types, dedicated to specific tasks such as mating with another cell. They sense their environment with chemical signalling systems, some of which are similar to those multicellular organisms use to coordinate their cells’ behaviour. And they may detect and capture their prey with the same kind of sticky surface molecules that hold cells together in animals and other multicellular organisms.

So what started it? One idea is that clumping together helped cells avoid being eaten by making them too much of a mouthful for single-celled predators. Another is that single cells are often constrained in what they can do – for example, most cannot grow flagella to move and also divide at the same time. But a colony can both move and contain dividing cells if each cell in it takes its turn.

Researchers are now trying to reconstruct the biology of the first multicellular creatures by studying the genomes of their nearest living relatives. “We’re trying to peer back hundreds of millions of years,” says Nicole King, a molecular biologist at the University of California, Berkeley. She and her team are studying single-celled protozoans called choanoflagellates to understand how animals came to evolve from them some 600 million years ago. Choanoflagellates and sponges – the only surviving witnesses to this step – share a common ancestor and King has found that choanoflagellates have a surprising number of equivalents to the signalling and cell-adhesion molecules unique to animals.

Yet bigger and more complex isn’t necessarily better. As King points out, unicellular life still vastly outnumbers multicellular life in terms of both biomass and species numbers. “So you could say unicellular life is the most successful, but that multicellular life is the most beautiful and dramatic.”

Claire Ainsworth

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