Two Domains, Then a Third
The first organisms were prokaryotes: no nucleus, no membrane-bound organelles, DNA in a single circular chromosome. They fall into two domains that are genuinely distinct, Bacteria and Archaea, differing in membrane lipids, cell-wall chemistry and transcription machinery. Archaea are, in several respects, closer to eukaryotes than to bacteria.
Prokaryotes reproduce by binary fission, an efficient doubling: one cell becomes 2, then 4, then 8. After divisions a single cell has become , so 20 divisions produce over a million. That exponential is why bacterial populations can respond to opportunity so fast.
They also exchange genes horizontally — by conjugation, transformation or transduction — rather than only passing them down. This is why antibiotic resistance can spread between unrelated species, and why the early tree of life is better drawn as a web than as a branching tree.
Eukaryotes appear later and are far larger and more complex. The endosymbiotic theory, established by Lynn Margulis, explains their organelles: mitochondria and chloroplasts descend from free-living bacteria engulfed and retained rather than digested. The evidence is strong and independent:
- Both have double membranes, as expected from engulfment
- Both retain their own circular DNA
- Both have 70S ribosomes, the bacterial type, not the eukaryotic 80S
- Both divide by binary fission, independently of the host cell
- Both are inhibited by antibiotics that target bacteria
Endosymbionts lose most of their genome over time, transferring genes to the host nucleus. A free-living bacterium carries a few thousand genes; a modern mitochondrion retains only around 37.
Viruses sit outside all of this. They have genetic material and a protein coat but no metabolism and no ribosomes, and they replicate only inside a host cell, which is why whether they are alive at all remains genuinely contested.
Common pitfall: treating the double membrane as proof on its own. It is suggestive, but the case rests on the convergence of independent lines, bacterial ribosomes, circular DNA, independent division and antibiotic sensitivity all pointing the same way.