Origin and Evolution of Early Cells

The First Cells: Cell Evolution

All organisms that currently inhabit the Earth are related because they come from ancestral cells that emerged more than 3,800 million years ago as a result of a slow evolutionary process of organic molecules accumulated in the primeval oceans. It is believed that the decisive moment in the origin of cells was the appearance of a biological membrane.

• The membrane separated the external environment from the internal environment, which favored the existence of a rudimentary metabolism that enabled the ancestral cells to get energy (via nutrition) and use it to reproduce and respond to environmental changes.

Because they originated in a sea of organic molecules, the first cells were probably anaerobic fermentative heterotrophic bacteria, able to obtain food and energy directly from their environment. But a situation like this is self-limiting, and these primitive creatures would have disappeared when food stocks ran out, if not for those first cells that evolved to give rise to cyanobacteria, which developed their own mechanism of energy production: photosynthesis.

The release of oxygen resulting from photosynthesis transformed the reducing early atmosphere into a new oxidizing atmosphere very similar to today’s. This oxygen gradually became a deadly poison for anaerobic organisms. Many of the existing cells disappeared; others took refuge in areas inaccessible to oxygen and have survived to this day. Others, however, managed to adapt and learned to use it in their metabolic reactions. Thus, primitive aerobic heterotrophic bacteria were able to use atmospheric oxygen to obtain energy from organic nutrients through a chemical process called cellular respiration, which releases carbon dioxide as a residual product.

Later, at least 2,700 million years ago, eukaryotic cells evolved from a symbiotic partnership between different prokaryotic bacterial cells with simpler organization, according to the endosymbiotic theory proposed by biologist Lynn Margulis.