Animal cells differ from single-cell organisms in two significant ways: 1) They don’t have the tools to survive outside an animal body 2) They are programmed to die after a certain number of cell divisions.
We know that it is possible for animal cells to live as individuals. The ancestor of all animals is believed to be the single-celled Choanoflagellata
From In Choanoflagellates, Clues to the Animal Kingdom’s Birth
Cells very similar to Choanoflagellates are found in the most primitive (meaning most like the original) animals such as sponges and ctenophores. DNA data suggest a family tree that looks something like this:
From Re: Why are sponges Animals, but colonial choanoflagellates not?
Once cells formed colonies and then true multicellular organisms they began losing genes needed to survive as unicellular organisms. Why? Because mutations in those genes would not affect their survival. Mutations are inevitable. Unless they are eliminated (by death) they will accumulate in a population. Useless genes become ghosts.
Animal cells die quickly in the wild. Under special conditions – such as a Petri dish where they are fed nutrients and growth factors and are protected from predators – animal cells can indeed live and grow as individuals. But just for a while.
Normal human cells will divide only about 50 times, a phenomenon known as the Hayflick Limit. This limit is due to the mechanics of chromosome replication. For reasons we don’t need to go into here, the ends of linear DNA molecules can’t be replicated. They have special caps of non-essential DNA known as telomeres, and a bit of the telomere is lost on every replication event. Once the telomeres are gone, the chromosomes can’t be replicated any more.
Why would there be a limit on replication? The most likely answer is to prevent uncontrolled growth of animal cells – in other words, cancer. The role of telomeres and the enzymes (telomerases) that can lengthen them is complicated [1] , but they do seem to act as a brake on cancerous growth.
Cells that enjoy the advantages of a multicellular life inevitably give up their ability to live independently. Human beings, as highly social animals, are much the same: we thrive in groups, but quickly deteriorate in isolation. Whether we are talking about cells or people, once you go down the path of sociability there is no turning back.
Footnotes