Cell structure

All eukaryotes probably share one unicellular ancestor that already had a nucleus and reproduced sexually. A host cell took in an aerobic bacterium by endocytosis but did not digest it. The two came to depend on each other (mutualism), and over many generations the bacterium became a mitochondrion. Later, in the ancestor of algae and plants, an engulfed cyanobacterium became a chloroplast.

Evidence: both organelles have 70S ribosomes, naked circular DNA and a double membrane, are about the size of bacteria, and arise only by binary fission of existing organelles. Their DNA sequences are closest to alphaproteobacteria (mitochondria) and cyanobacteria (chloroplasts). The theory made predictions that sequencing later confirmed.

All the cells of an organism come from one zygote by mitosis, so they share the same genome. During differentiation different genes are expressed in different cells, triggered by signals from the environment and by the cell’s position. Each cell type therefore has its own proteome: a developing red blood cell makes large amounts of haemoglobin, a muscle cell makes actin and myosin. Specialized cells form tissues and organs and share the work, but each depends on the others.

All animals and plants, many fungi and many algae are multicellular. These groups are separated on the tree of life by unicellular ones, so multicellularity evolved independently many times. It allows a larger body and specialized cells, but needs cells that stick together, signal to each other and differentiate. Volvox colonies and choanoflagellates, the closest living relatives of animals, show how the change may have begun.