Multicellular Organisms

Multicellular Organisms

In the history of the universe, one day, a molecule arose that was able to make crude copies of itself, using as building blocks other molecules in the organic soup of gradually increasing complexity.

This was the earliest ancestor of deoxyribonucleic acid, DNA, the master molecule of life on Earth. It is shaped like a ladder twisted into a helix, the rungs available in four different molecular parts, which constitute the four letters of the genetic code. These rungs, called nucleotides, spell out the hereditary instructions for set of instructions, written out in essentially the same language. The reason organisms are different is the differences in their nucleic acid instructions.

A mutation is a change in a nucleotide, copied in the next generation, which breeds true. Since mutations are random nucleotide changes, most of them are harmful or lethal, coding into existence non-functional enzymes. It is a long wait before a mutation makes an organism work better. And yet it is that improbable event, a small beneficial mutation in a nucleotide a ten-millionth of a centimeter across, that makes evolution go.

Multicellular organisms. You and me, we are made of a hundred trillion cells. We are, each of us, a multitude. Gorgeous Pattern Texture. Image © Megan Jorgensen (Elena)

Four billion years ago, the Earth was a molecular Garden of Eden. There were as yet no predators. Some molecules reproduced themselves inefficiently, competed for building blocks and left crude copies of themselves. With reproduction, mutation and the selective elimination of the least efficient varieties, evolution was well under way, even at the molecular level. As time went on, the got better at reproducing. Molecules with specialized functions eventually joined together, making a kind of molecular collective – the first cell.

Plant cells today have tiny molecular factories, called chloroplasts, which are in charge of photosynthesis – the conversion of sunlight, water and carbon dioxide into carbohydrates and oxygen. The cells in a drop of blood contain a different sort of molecular factory, the mitochondrion, which combines food with oxygen to extract useful energy. These factories exist in plant and animal cells today but may once themselves have been free-living cells.

But three million years ago, a number of one-celled plants had joined together, perhaps because a mutation prevented a single cell from separating after splitting in two. The first multicellular organisms had evolved. Every cell of your body is a kind of commune, with once free-living parts all banded together for the common good.