The amazing circle of life

The amazing circle of life
Paul Davies

Over the past two centuries scientists have painstakingly pieced together the history of life. The fossil record shows clearly that ancient life was very different from extant life. Generally speaking, the farther back in time you go, the simpler were the living things that inhabited Earth. The great proliferation of complex life forms occurred only within the last billion years. The oldest well-documented true animal fossils, also to be found in Australia (in the Flinders Mountains north of Adelaide), are dated at 560 million years. Known as ediacara, they include creatures resembling jellyfish.

Shortly after this epoch, just about 545 million years ago, there began a veritable explosion of species, culminating in the colonisation of the land by large plants and animals. But before about one billion years ago, life was restricted to single-celled organisms. This record of complexification and diversification is broadly explained by Darwin's theory of evolution, which paints a picture of species continuallybranching and rebranching to form more and more distinct lineages. Conversely, in the past these lineages converge.

The evidence strongly affirms that all life on Earth descended via this branching process from a common ancestor. That is, every person, every animal and plant, every invisible bacterium, can be traced back to the same tiny microbe that lived billions of years ago, and thence back to the first living thing. What remains to be explained what stands out as the central unsolved puzzle in the scientific account of life is how the first microbe came to exist.

Peering into life's innermost workings serves only to deepen the mystery. The living cell is the most complex system of its size known to mankind. Its host of specialized molecules, many found nowhere else but within living material, are themselves already enormously complex. They execute a dance of exquisite fidelity, orchestrated with breathtaking precision. Vastly more elaborate than the most complicated ballet, the dance of lifeencompasses countless molecular performers in synergetic coordination. Yet this is a dance with no sign of a choreographer.

No intelligent supervisor, no mystic force, no conscious controlling agency swings the molecules into place at the right time, chooses the appropriate players, closes the links, uncouples the partners, moves them on. The dance of life is spontaneous, self-sustaining and self-creating.

How did something so immensely complicated, so finessed, so exquisitely clever, come into being all on its own? How can mindless molecules, capable only of pushing and pulling their immediate neighbours, cooperate to form and sustain something as ingenious as a living organism?

Solving this riddle is an exercise in many disciplines biology foremost but chemistry, geology, astronomy, mathematics, computing and physics contribute too. It is also an exercise in history. Few scientists believe that life began in a single monumental leap. No physical process abruptly `breathed life' into inertmatter.

There must have been a long and complicated transitional stage between the non-living and the first truly living thing, an extended chronology of events unlikely to be preordained in its myriad details....

Excerpted from `The Fifth Miracle the Search for the Origin of Life' by Paul Davies; Penguin; Pounds 5.99