The Universal Urge To Join Up
Take a mouse cell and place it in contact with a human cell. The two separating membranes will dissove and the cell contents will mix. The once-independent and widely different cell nuclei will fuse, forming a single hybrid cell with a common membrane. Even more astonishing, this totally new biological entity will often divide and produce an endless line of the new hybrid. As might be expected, some hybrids do not remain true and revert to one or the other of the original species.
Although cell fusion has been observed only under laboratory conditions, it seems to represent a near-universal cell phenomenon that might be realized rarely under natural conditions. The implications for the history of life are far-reaching. For example, the mitochondria in human cells that help our bodies use oxygen to obtain energy may well be descendants of bacteria that once fused with primitive cells. The same may be true for the chloroplasts in plant cells.
(Thomas, Lewis; "Cell Fusion: Does It Represent a Universal Urge to 'Join Up'?" Science Digest, 86:52, December 1979.)
Comment. Natural cell fusion might make large evolutionary steps possible and be much faster than endless small genetic changes. Are we all composite creatures?