The Sky is Made by Life

The Sky is Made by Life

Life had arisen almost immediately after the origin of the Earth, which suggests that life may be an inevitable chemical process on an Earth-like planet.

Sex seems to have been invented around two billion years ago. Before then, new varieties of organisms could arise only from the accumulation of random mutations, the selection of changes, letter by letter, in the genetic instructions. Evolution must have been agonizingly slow. With the intervention of sex, two organisms could exchange whole paragraphs, pages and looks of their DNA code, producing new varieties ready for the sieve of selection. Organisms are selected to engage in sex – the ones that find it uninteresting quickly become extinct. And this is true not only of the microbes of two billion years ago. We humans also have a palpable devotion to exchanging segments of DNA today.

By one billion years ago, plants, working cooperatively, had made a stunning change in the environment of the Earth. Green plants generate molecular oxygen. Since the oceans were by now filled with simple green plants, oxygen was becoming a major constituent of the Earth’s atmosphere, altering it irreversibly from its original hydrogen-rich character and ending the epoch of Earth history when the stuff of life was made by non-biological processes.

Perhaps there are many other planets that today have abundant microbes but no big beasts and vegetables. Image Dark Colored Mosaic: Megan Jorgensen (Elena)

But oxygen tends to make organic molecules fall to pieces. Despite our fondness for it, it is fundamentally a poison for unprotected organic matter. The transition to an oxidizing atmosphere posed a supreme crisis in the history of life, and a great many organisms, unable to cope with oxygen, perished. A few primitive forms, such as botulism and tetanus bacilli, manage to survive even today in oxygen-free environments. The nitrogen in the Earth’s atmosphere is much more chemically inert and therefore much more benign than oxygen. But it, too, s biologically sustained. Thus, 99 percent of the Earth’s atmosphere is of biological origin.

For most of the four billion years since the origin of life, the dominant organisms were microscopic blue-green algae, which covered and filled the oceans. Then some 600 million years ago, the monopolizing grip of the algae was broken and an enormous proliferation of new lifeforms emerged, an event called the Cambrian explosion.

But life did not evolve much beyond blue-green algae for three billion years, which suggests that large lifeforms with specialized organs are hard to evolve, harder even than the origin of life.

If we plunged through a pore into a nucleus of the cell, we would find something that resembles an explosion in a spaghetti factory – a disorderly multitude of coils and strands, which are the two kind of nucleic acids: DNA, which knows what to do, and RNA, which conveys the instructions issued by DNA to the rest of the cell.

These are the best that four billion years of evolution could produce, containing the full complement of information of how to make a cell, a tree or a human work.

A fertilized egg takes many as long to wander from the fallopian tubes and implant itself in the uterus as Apollo 11 took to journey to the Moon; and as long to develop into a full-term infant as Viking took on its trip to Mars. The normal human lifetime is longer than Voyager will take to venture beyond the orbit of Pluto.