Mostly added disease.

Added ability to fall and hurt oneself.
Added ability to infect others with disease, and infect water and waste.
Added ability to die, and an energy cost of living.
Diseases cost energy, gradually get stronger and then weaker.
Diseases can spread from randomness to person, person to person, person to environment, and
environment to person.
Added genetic and age related slight immunity from diseases.
There have been more. Look at code for details.