Top 10: Life’s greatest inventions
11:38 09 April 2005 by Rachel Nowak
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2. The eye
3. The brain
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 complex cells that package up their DNA in a nucleus – took things to a new level. They formed 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. Back to top
2. The eye
THEY appeared in an evolutionary blink and changed the rules of life forever. Before eyes, life was gentler and tamer, dominated by sluggish soft-bodied worms lolling around in the sea. The invention of the eye ushered in a more brutal and competitive world.