Supernovae and Life

Part 8 - Cells

The development of a protective protein envelope, which evolved into the cell wall and membranes of modern microbes, was a major improvement. It provided protection from the environment, permitting the organism to develop in a less disruptive space and keeping out larger molecules that could be dangerous invaders. The cell wall had to be porous enough to pass small molecules, allowing nutrients like carbon dioxide to enter and waste products to exit. Keeping the tiny molecules of hydrogen from leaking through the cell wall while they were being moved within the cell was solved by attaching them to a much larger molecule for the journey. Getting more nutritious food was solved by engulfing smaller cells whole.

Microbes reproduce by producing a replica of the original within the cell and then dividing the cell into two identical parts. Cells are able to evolve only by random errors in the genetic pattern or by absorbing genetic material from other microbes or viruses, a process known as horizontal gene transfer.

There is evidence of photosynthesis, around 3.5 billion years ago. Bacteria needed hydrogen and carbon to grow and began using sunlight to break down water and the carbon dioxide dissolved in the water. They released the poisonous waste product, oxygen, into the environment.

There was no significant amount of free oxygen in the atmosphere or the oceans until about 2.4 billion years ago, so early life was anaerobic (It did not use oxygen). The tiny cells were unable to propel themselves and depended on the environment to provide food and remove waste products.

All microbes lived in a consortium of co-operating organisms where one group might fix nitrogen and another carbon. Waste molecules produced by one organism were often food for another so the microbes generally lived in mutually beneficial consortia of unrelated organisms.

Living in close quarters and dependent on each other for nutrient required a system of communication. This involved the release of molecules which floated around until they attached themselves to a specific site on another microbe's surface. This then instructed the cell release or absorpt other molecules in a process known as quorum sensing. This also served to keep them together and to deter hostile non-members.

This generally resulted in a rough balance of nutrients but usually there was a release of gases (typically oxygen, carbon dioxide, sulfur dioxide and nitrogen) that were carried away by wind or current and might be used by other consortia.

In 1977, Carl Woese and George Fox, biochemists at the University of Illinios, reported that all organisms could be classified, on the basis of subtle differences in the sequences of ribonucleic acid molecules in their ribosomes, into only three categories:- eubacteria, archeabacteria and eucaryotes. They also observed that all living organisms could be arranged on a tree of life based on the way each organism made protein.

On the tree of life, plants and animals represent only a few twigs and the closest genetic relatives to animals, including us, are fungi. The overwhelming majority of life on Earth is microbial. There are at least several million species of microbes, far more than all animals and plants combined. And their ancestors were living on Earth for more than three billion years before plants and animals evolved.

A 2016 study, comparing the genomes of organisms living today, identified 355 genes believed to have been present in the last universal common ancestor of all current life on Earth (LUCA). It was a small, single-celled organism with a ring-shaped coil of DNA floating freely within its cell, that lived 3.5 to 3.8 billion years ago. It was already a complex life form with many ways to convert information between DNA, RNA, and proteins. Some of those genes, however, could have been acquired later by horizontal gene transfer between Archaea and Bacteria, the two original microbial families.