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Evolution
The processes of parasite evolution are still largely a mystery. Scientists theorize that hookworms were once free-living organisms and evolved a dependence on some mammals to complete their life cycle. Hookworms likely evolved a parasitic lifestyle after the evolution of their hosts.
Becoming a Parasite
Hookworms may have developed a parasitic relationship with their hosts when humans domesticated animals. Dogs and pigs likely aided in this process by swallowing and passing hookworm eggs into soil where the larvae could develop and infect barefoot humans.
Hookworm Breeding Ground
When small, nomadic groups of early humans settled into larger, sedentary villages they created ideal conditions for the survival of hookworm larvae outside the hosts' body and an effective mechanism for the infective larvae to enter the hosts. Once larvae penetrate the skin, they are carried away through the bloodstream to the lungs.
Immune Response
Hookworms evolved mechanisms for infection to counter the immune response in the host. The host's immune system produces mucus in response to the hookworms' presence in the lungs. Coughing provides the larvae transport to the mouth where they can be swallowed to complete their life cycle in the host's gut.