What Makes Evolution Go

What Makes Evolution Go

The Earth condensed out of interstellar gas and dust some 4.6 billion years ago. We know from the fossil record that the origin of life happened soon after, perhaps around 4.0 billion years ago, in the ponds and oceans of the primitive Earth.

The first living things were not anything so complex as a one-celled organism, already a highly sophisticated form of life. The first stirrings were much more humble. In those early days, lightning and ultraviolet light from the Sun were breaking apart the simple hydrogen-rich molecules of the primitive atmosphere, the fragments spontaneously recombining into more and more complex molecules. The products of this early chemistry were dissolved in the ocean, forming a kind of organic soup of gradually increasing complexity, until one day, quite by accident, a molecule arose that was able to make crude copies of itself, using as building blocks other molecules in this planetary soup.

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 foru letters of the genetic code.

Four different molecular parts – four letters of genetic code. One may understand the cosmos, but never the ego; the self is more distant than any star. Gilbert K. Chesterton. Photograph by Elena.

These rungs, called nucleotides, spell out the hereditary instructions for making a given organism, and every lifeform on Earth has a different 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.

Cambrian Explosion

On the Earth, before the Cambrian explosion species seem to have succeeded one another rather slowly. In part this may be because the richness of our information declines rapidly the farther into the past we peer. In fact, in the early history of the planet Earth, few organisms had hard parts and soft beings leave few fossil remains.

In part the sluggish rate of appearance of dramatically new forms before the Cambrian explosion is real; the painstaking evolution of cell structure and biochemistry is not immediately reflected in the external forms revealed by the fossil record.

The forest. Photograph by Elena.

After the Cambrian explosion, thus exquisite new adaptations followed one another with comparatively breathtaking speed. In rapid succession, the first fish and the first vertebrates appeared; plants, previously restricted to the oceans, began the colonisation of the land; the first insect evolved. Its descendants became the pioneers in the colonisation of the land by animals; winged insects arose together with the amphibians, creatures something like the lingfish, able to survive both on land and in the water.

The first trees and the first reptiles appeared… the dinosaurs evolved, the mammals emerged, and then the first birds; the first flowers…

The dinosaurs became extinct and the earliest cetaceans, ancestors to the dolphins and whales, arose and in the same period the primates – the ancestors of the monkeys, the apes and the humans.

(By Carl Sagan, Cosmos)