Happenstance and Evolution

Happenstance and Evolution

Were the Earth to be started over again with all its physical features identical, it is extremely unlikely that anything closely resembling a human being would ever again emerge, because there is a powerful random character to the evolutionary process. A cosmic ray striking a different gene, producing a different mutation, can have small consequences early but profound consequences late.

Happenstance may play a powerful role in biology, as it does in history. The father back the crucial events occur, the more powerfully can they influence the present.

Thus, any given organism is the way it is because of a long series of individually unlikely steps. I do not think life anywhere else would look very much like a reptile, or an insect or a human – even with such minor cosmetic adjustments as green skin, pointy ears and antennae. But if you pressed me, I could try to imagine something rather different:

On a giant gas planet like Jupiter, with an atmosphere rich in hydrogen, helium, methane, water and ammonia, there is no accessible solid surface, but rather a dense, cloudy atmosphere in which organic molecules may be falling from the skies like manna from heaven, like the products of a chemical laboratory experiments. However, there is a characteristic impediment to life on such a planet: the atmosphere is turbulent, and down deep it is very hot. An organism must be careful that it is not carried down and fried.

Each Star System is an Island in Space, but it is also a Musical Creation. Image Purple Pretty © Elena

To show that life is not out of the question in such a very different planet, Carl Sagan and his Cornell colleague E. E. Salpeter have made some calculations. Of course, they cannot know precisely what life would be like in such a place, but they wanted to see if, with the laws of physics and chemistry, world of this sort could possibly be inhabited.

One way to make a living under these conditions is to reproduce before you are fried and hope that convection will carry some of your offspring to the higher and cooler layers of the atmosphere. Such organisms could be very little. Sagan and Salpeter called the sinkers. But you could also be a floater, some vast hydrogen balloon pumping helium and heavier gases out of its interior and leaving only the lightest gas, hydrogen; or a hot-air balloon, staying buoyant by keeping your interior warm, using energy acquired from the food you eat. Like familiar terrestrial balloons, the deeper a floater is carried, the stronger is the buoyant force returning it to the higher, cooler, safer regions of the atmosphere.

A floater might eat preformed organic molecules, or make its own from sunlight and air, somewhat as plants do on Earth. Up to a point, the bigger a floater is, the more efficient it will be. Salpeter and Sagan imagined floaters kilometers across, enormously larger than the greatest whale that ever was, beings the size of cities.