Earth is experiencing its "sixth great extinction event" with disease and human activity taking a devastating toll on vulnerable species, according to a major review by conservationists.

The New England study reveals that the children of centenarians are less than one-third as likely to die of cancer as the general population, and less than one-sixth as likely to die of heart disease.

The sixth leading cause of death was Alzheimer's disease, one of six diseases or causes for which the age-adjusted death rate increased significantly, the study says.