Mars Rhapsody, Chapter 15 Evolution

Life has one imperative, one that essentially defines life: reproduction. Something that doesn’t grow and doesn’t make copies of itself is not life. To take raw resources and make copies of itself requires an extremely complex object. That very complexity means that the copying process may make mistakes for a wide variety of reasons. Mistakes mean change.

If you make a random change to anything complex, you’re likely to break it. If you make billions of changes, many will be innocuous and some may give it an advantage over its forbears. In those early seas, life must have been very simple compared to what we see today while still very complex compared to ordinary chemicals and minerals.

The first self-reproducing systems were almost definitely not cells but rather large molecules. Those molecules had to be fragile. Eventually, they associated with other, smaller molecules that could surround them with a protective coating without preventing reproduction. This association must have been the result of a fortuitous change in the molecule that attracted the smaller, protective molecules. While you'd hardly call this clump of molecules a cell, it was the template for cellular life.

Millions of years and billions of experimental changes later, the first, barely viable simplest possible cells developed. They grew by accretion of chemicals in the water, by adding one chemical at a time to a copy under manufacture in the original cell. They had the means to add onto their protective coating. Eventually, the whole became large enough to split. Most such splits failed but many worked by chance — enough so that this first cellular species multiplied.

From here, it was merely a matter of time. The normal process of making mistakes in copying caused a rare change that made the entire process better for reproduction, for the imperative of life. We will never see these original life forms in ancient rocks on Earth because they were outcompeted by their descendants and their organic materials recycled. They were erased as thoroughly and completely as a washed blackboard or a whiteboard sprayed with whiteboard cleaner and carefully wiped dry. No shadow or hint remains of the beginnings of life on Earth. The distant moons of the gas giant planets have oceans of water deep beneath their surfaces. Some even spew forth their contents on the moon surfaces as geysers. There is a small possibility that these geysers contain very early, pre-cellular life and that robotic missions to the moons may be capable of analyzing it.

This entire process probably happened quickly in geological time, only millions or tens of millions of years. Mars had plenty of time for it. So did Venus. Obviously, Earth did. It’s likely that the same process has taken place millions of times in our galaxy and billions of times across the universe and may be playing out in extreme slow motion deep in underground oceans on those moons.

Life is life, and simple, single-celled life qualifies as well as any other. Once life had evolved to single-celled prokaryotes (simple cells without organelles such as a nucleus), there was little impetus to go further. These cells used available chemical energy sources to live their slow lives and used other chemicals to grow ever so slowly until they could split into daughter halves. Between the chemical reactions in the atmosphere and on the barren land and the cosmic rain of new chemicals, life could go on indefinitely at its glacial pace. But it didn’t.

Either a new stress or newly abundant supplies of food chemicals could have triggered a change. Many theorize that it was the former in the form of extensive glaciation, possibly even complete coverage of the entire Earth with ice, that upset the business-as-usual single-celled life on Earth. Life became very difficult in this scenario, and few cells survived. Among those that did were some with special characteristics. Without food, some cells could use sunlight as an energy source. This accidental change allowed those little green cells to take off when the Earth thawed again. The plentiful carbon dioxide, the warming temperatures, and the abundant sunlight made perfect conditions for this new cellular species born of the harsh ice climate.

These new photosynthetic cells caused a crisis of life by producing toxic oxygen as a byproduct of their life processes. Some cells adapted to the slowly increasing oxygen concentrations in the air. Others remained anaerobic, and some of those can be found today. Oxygen in the atmosphere led to new metabolic pathways and, ultimately, to multicellular creatures that could not exist using the leisurely lifestyles of the early Earth.

Legs and eyes appeared in these multicellular creatures who began to search for their food instead of just taking in whatever they bumped into. Hard skins came next to thwart those who would devour. Teeth appeared to pierce the hard skins that next became shells. This all took place so fast in geological terms that it is called the Cambrian explosion, around 540 million years ago.

The photosynthesizers were made into partners for some of the new eukaryotes (cells with internal organelles such as chloroplasts that may be the remnants of photosynthesizers absorbed but not digested by other cells) and then became the multicellular version we call plants. Plant growth exploded during the Carboniferous period that lasted for about 60 million years beginning about 360 million years ago.

Massive amounts of plant material were buried during this period. The bark of those trees contained lots of lignin and formed the massive veins of coal found around the world.

So it was that four billion years of evolution on Earth, with conditions favorable to evolution, was required to create the coal that fueled the Industrial Revolution. Mars was dead long before that time and so has no veins of coal, no oil, and no natural gas. Any single-celled life on its surface would have long ago been destroyed by the thin atmosphere, the dryness, the intense ultraviolet light, and the cosmic and solar radiation constantly bombarding its surface.