Human lifespan has increased considerably since the 1600s, when the average lifespan was 30 years. By 2012, the average US life expectancy was nearly 79. Reasons for the increase include sewers and other sanitation measures, antibiotics, clean water, refrigeration, vaccines and other medical efforts to prevent children and babies from dying, improved diets, and better health care.

Some scientists predict average life expectancy will continue to increase, although many doubt the average will ever be much higher than 90. But a few say vastly longer lifespans are possible.

Cawthon says that if all processes of aging could be eliminated and oxidative stress damage could be repaired, "one estimate is people could live 1,000 years."