Origin Story: A Big History of Everything

by David Christian

eukaryotes had one more trick up their sleeve: sex. Like all species, prokaryotes pass their genes on to their offspring. Most just split in two and pass on their genes through asexual reproduction. But, as we have seen, prokaryotic genes can also travel sideways as bits of DNA and RNA jump ship, go on the road, and find new homes inside other cells. Prokaryotic cells share genes the way humans share library books. But eukaryotes have a different and more complex way of passing on their genes, and they pass them on only to their offspring, never to strangers. In eukaryotes, the genetic material is locked inside the protected vault of the nucleus. That material is released only under the most stringent conditions, using rules less promiscuous and more orderly than those of prokaryotes, and these rules affect how eukaryotic cells evolve. When eukaryotes produce germ cells—eggs and sperm, the cells from which their offspring will be formed—they don’t just copy their DNA. They stir it around first. They swap some of their genetic material with another individual of their species so that the offspring of the two parents gets a random selection of genes, one half from one parent and the other half from the other parent. Both the genetic and the physical mechanisms involved in this elaborate dance are exquisitely complex. But the result was to add a new twist to evolution. Slight but random genetic variations were guaranteed every generation, because even if most of the genes wer...