We Eukaryotes

Part 1 - Eukaryotes

Author's note. This book is one of a series originally published as Who the Hell are We? but extensively revised and updated.

Thank you for reading, voting, following and adding, 'We Eukariotes' to your reading list or library.   Dunc MacPhun   2022 March 24. 

In chronological order, the series contains:-

https://www.wattpad.com/myworks/234080674-supernovae-and-life

https://www.wattpad.com/myworks/238813918-we-eukaryotes

https://www.wattpad.com/myworks/244084318-neolithic

https://www.wattpad.com/myworks/247058691-our-sea

https://www.wattpad.com/myworks/249120741-migration

https://www.wattpad.com/myworks/251168052-middle-era

https://www.wattpad.com/myworks/254532133-disease

https://www.wattpad.com/myworks/256201647-atoms-light

https://www.wattpad.com/myworks/257785133-steam

I will be publishing more work in this series.

The first living organisms appeared on Earth about 3.5 billion years ago, evidenced by Stromatolites formed by cyanobacterial mats in Western Australia (shown on the cover photograph) but the first multicellular animals did not appear until about 650 million years ago.

During that vast amount of time, life was exclusively microbial and most of it was living in the seas (although recent evidence suggests some bacteria may have lived on land 3.2 billion years ago). It consisted of virus like particles and single cell organisms, many living together in symbiotic relationships.

About 1.5 billion years ago, the eukaryotes, while still single-celled organisms, became the common ancestors of all multicellular plants, animals, and fungi.  And, about 1.2 million years ago, these eukaryotes evolved sexual reproduction (possibly as a way to repair damage to genetic information) thereby greatly increased the speed of evolution.

Until the evolution of sexual reproduction, evolution was a hit or miss process. Cell division was the only method of producing offspring and these were identical to the original, except for an occasional error in genetic information or if the cell acquired genetic information by horizontal gene transfer from other organisms. A virus might invade a cell or one microbe might engulf another cell and some of the genetic information might get stuck there.

Genes borrowed from viruses may have been vital to the differentiation of multicellular tissues and even the fusion of sperm with egg cells. (Fusogen EFF1 is a viral components of cells which links one cell to another in viral infections. The fact that all known cell fusion molecules have a viral origin suggests they were also important in the evolution of inter-cellular communication systems).

There were also evolutionary advantages to being part of a multicellular organism. It was not limited to the life of one cell but continued to live even as individual cells died and were replaced by cell division.   It could consume smaller organisms more easily and was less likely to be consumed or damaged by single celled organisms.   It could develop specialized cells, such as flagellates as a way to move around or to move gases and nutrients past the cells that needed them, and produce chemicals to deter potential predators.

But sticking free floating, single cells together required the evolution of the proteins integrin, collagen, the cadherins and other trans-membrane proteins that were essential for cell signalling in all multicellular animals. This was a big evolutionary advantage as it allowed cells to form permanent consortia (mats attached to rocks and sand and free floating communities). This permitted reliable supply of nutrients from unrelated organisms and removal of waste products.

(Collagen is the main component of connective tissue and the most abundant protein in mammals, making up from 25% to 35% of the protein content of the body. It is mostly found in fibrous tissues such as tendons, ligaments, and skin. Depending upon the degree of mineralization, collagen tissues may be rigid (bone) or varying degrees of compliant (tendon and cartilage). It is also found in blood vessels, the gut, corneas, spinal discs and the dentin in teeth).

Many other problems were gradually solved by the evolution of thousands of other proteins and molecules including:-

The proteins actin and myosin that worked as nano-motors to power single celled eukaryotes (and to propel sperm and move food through guts in all modern organisms) these evolved into the muscles of all animals.

Elastin an elastic protein provided flexibility to connective tissue.

Keratin is one of a family of fibrous structural proteins found in hair, nails, feathers, horns, claws, hooves, calluses, and the outer layer of vertebrate skin. (Spider silk is classified as keratin, although this protein probably evolved independently of the process in vertebrates).

Chitin is the main component of the wings and exoskeletons of arthropods, such as crustaceans and insects, cell walls in fungi, and the scales of fish.

The size of single cells was limited because of the difficulty of getting gases (oxygen, carbon dioxide etc) and nutrients to all parts of the cell. The problem was similar when cells started to clump together, the cells at the bottom got very little oxygen.

The problem of getting oxygen into all the cells was a initially solved by the cells forming into thin mats attached to rocks or by tubular or balloon shapes with all cells being in direct contact with the water. Modern versions of this body plan are jellyfish. The increase in atmospheric oxygen, produced by eukaryotic phytoplankton, benefited later organisms as oxygen was more readily available to filter through several layers of cells. Apart from free floating single celled organisms and viroids, the main form of early organized life were mats formed by coexisting bacteria and archaea glued to rocks or sand.

Scientist have identified 6,331 groups of genes common to all living animals that were probably derived from a single common ancestor. The Hox genes (important in body plan development and found in the Placozoa and the higher animals) probably evolved about 760 million years ago, about the time multicellular life began to diverge. The first to split off were the sponges (porifora) 769 million years ago followed by Placozoa (the thin plate-like organism which may have been the ancestor of all the animals) 680 million years ago.

The earliest forms of life on the planet had no choice but to assemble every molecule in their cells from basic elements and chemical compounds, collected the from the environment, using a power source such as the Earth's heat or radiation from the sun.

When they died, many of the complex molecules they had made were released into the environment and were used by newer organisms. Over time some of them lost the ability to make their own molecules with basic ingredients and lived by consuming the waste product of others and this included dead organisms.

These microbes (and later the fungi) became the garbage collectors of the world which explains why the planet is not covered, kilometres deep, in dead animals.  Fortunately, they are with us still.

Other organisms did not find enough dead organisms in the garbage and set about killing live organisms. They became the first predators.