Earth Science: Origins Of Life – First Living Cells
Over the past 3.5 billion years, living things have developed from simple cell structures to complex multicellular and higher organisms.
The first cell-like structures were the protobionts, from which it was a clear path to prokaryotes-the first true cells.
Prokaryotes: the first cells
The first prokaryotes were very similar to today’s bacteria and cyanobacteria. While the prokaryotes lacked a true nucleus, they had a cell wall that separated them from the environment and provided an internal space in which metabolic processes could take place.
Some early prokaryotes used sunlight for their energy needs. They used an oxygen-free (anaerobic) form of photo- synthesis, oxidizing hydrogen sulfide and giving off sulfur as a waste product. Today, purple bacteria still use this type of energy production. The next step was the adoption of oxygen-producing photosynthesis.
Using energy from the sun, carbon dioxide, and the surrounding water supply, cyanobacteria created their own nutrients. Oxygen-which was poisonous to other living things at the time—was released as a waste product. With the success of these organisms, the prokaryotes that used anaerobic photosynthesis retreated to oxygen-free habitats (such as sulfurous hot springs), leaving the way open for oxygen-breathing organisms.
The oldest known prokaryote fossils are stromatolites, which date back approximately 3.4 billion years. Stomatolites are calcium carbonate, or chalk, deposits that form colonies of cyanobacteria.
The first eukaryotes
About two billion year ago, the first eukaryotes appeared. They were probably the result of symbiotic relationships among prokaryotic cells. They had a true cell nucleus and formed the foundation for the development of higher organisms.
All single-celled organisms and all multi- cellular living things-plants, animals, and humans—are eukaryotes.
Multicellular organisms
It is probable that multicellular life may have started in colonies of single-celled organisms in which daughter cells did not separate from others following cell division Or they could have emerged from single- celled organisms with multiple nuclei. The Precambrian period (roughly 600 to 700 million years ago) probably saw the emergence of the first multicellular organisms.
There is little fossil record of these early organisms, since they did not have hard physical components (such as shells). However, the imprints of soft-bodied organisms of the Ediacaran period can be found in many locations. Later, in the early Cambrian period, many new organisms developed, during a 50 million year period known as the Cambrian Explosion.
The first plants came ashore about 400 million years ago, pioneering the colonization of dry land by living organisms.
BASICS
END0SYMBI0NT THEORY proposes that eukaryotic cells emerged through the fusion of prokaryotic cells into symbiotic communities that later became single organ- isms: eukaryotes.