Part I of this series describes how the Earth acquired oceans and other characteristics that make it highly suitable for life. The prebiotic molecules that help kick-start living organisms can be found throughout the universe hitchhiking on meteorites and comets. Here we look at how single-celled organisms, which occur naturally on a watery world, developed the ability to evolve into multicellular organisms, and much later, intelligent life.

Obstacle 4 to Intelligent Life – No Eukaryotes

The first primitive single-celled organisms to appear were prokaryotes, classified as bacteria and archaea. Prokaryotes have a problem. As they grow larger, their volume and energy needs increase much faster than their surface area. Since they obtain energy by interacting with their environment through their outer surface, they quickly reach a size limit and must stay tiny.

A chance solution to the energy problem resulted when a new cell type, the eukaryote, emerged. Unlike prokaryotes, a eukaryote has an internal membrane-bound nucleus and other internal organelles, one being a mitochondria, which provides far more energy than could be produced by the cell itself. Only single-celled organisms called eukaryotes have the energy and size to evolve into complex multicellular organisms. Without eukaryotes, intelligent life cannot evolve.

Photosynthetic bacteria which produce oxygen gas as a waste product evolved early in Earth's history. Eventually, the production of oxygen led to a world-wide catastrophe which forced other prokaryotes to mutate to be compatible with oxygen, or die.

The first eukaryote came about as a merger between two cell types, an anaerobic (oxygen incompatible) archaeon and an aerobic (oxygen compatible) bacterium. The aerobic bacterium became the mitochondria of the archaeon, and the merger became an alternative way for at least one species of archaea to survive oxygenation.

There was a relatively short time frame when oxygen stress was beginning to kill many prokaryotes, but before all anaerobic cells died. The merger came about in part because of a unique characteristics of the two cells.

At least one species of archaeon had developed a DNA mutation that provided greater flexibility of its cell membrane, allowing it to surround objects touching it, and pull them inside for ingestion. It also had folds in its outer skin to provide greater surface area. This innovation also allowed it to wall off internal structures like its nucleus.

When an aerobic bacteria slipped into a fold of the archaeon, possibly as a parasite, the archaeon engulfed it. The captured aerobic bacteria had its own unique DNA. It could perform a chemical exchange with the archaeon by consuming oxygen molecules, and supplying it with energy in the form of ATP molecules as a waste product.