Large organisms use small symbiotes to adapt quickly

One of the problems of being a large organism is that you evolve slowly. That's OK if your environment is static - but in a rapidly-changing environment, you can't evolve as quickly as smaller creatures can. A failure to adapt to a new environment can rapidly result in extinction.

The number one example of this is gut bacteria. The edible aspects of an organism's environment may undergo all kinds of fluctuation - including seasonal changes, and dramatic changes in habitat due to migration. Using gut bacteria allow microorganisms to evolve to traqck the habitat changes within their host's lifespan. Their shorter reproductive cycle allows them to evolve faster.

Humans use their symbiosis with gut bacteria to adapt to rapidly-changing aspects of their environment. They also use their symbiosis with memes to adapt to rapidly-changing aspects of their environment. The principle of a large organism using small symbionts to adapt quickly remains the same, however.