Darwin included the concept of soft inheritance in Origin, mentioning "variability from the indirect and direct action of the external conditions of life, and from use and disuse," for example. It has been said that Darwin himself was not a particularly strict Darwinian, meaning that his work allowed for a wider variety of mechanisms than many of his 20th-century followers would accept. "In a way," says Jablonka, "we're going back to Darwin and his original, much broader notion of heredity."
Origin barely touched upon the most contentious evolutionary issue: If all life has evolved from "lower forms," does that include people? Darwin finally addressed the issue in The Descent of Man, and Selection in Relation to Sex, published in 1871, explaining he had been studying human evolution for years, but "with the determination not to publish, as I thought that I should thus only add to the prejudices against my views." How right he was, both that "man is the modified descendant of some pre-existing form"—and that an awful lot of people would prefer to believe otherwise. They shared Disraeli's discomfort at being descended from apes and complained that evolution pushed a divine creator to the side.
Disbelief in human descent may have been a justifiable comfort in Darwin's time, when few fossils of human ancestors had been discovered, but the evidence no longer allows it. Darwin, in Origin, admitted that the lack of "intermediate varieties" in the geological record was "the most obvious and gravest objection which can be urged against my theory."
The objection certainly applied to the paucity of ancestral human fossils in Darwin's time. Years of painstaking work by paleontologists, however, have filled in many of the important gaps. There are many more extinct species to be discovered, but the term "missing link" has for the most part become as outdated as the idea of special creation for each species. Anthropologists once depicted human evolution as a version of the classic "March of Progress" image—a straight line from a crouching proto-ape, through successive stages of knuckle draggers and culminating in upright modern human beings. "It was a fairly simple picture, but it was a simplicity born of ignorance," says biological anthropologist William Jungers of Stony Brook University in New York. "The last 30 years have seen an explosion of new finds."
There are now hundreds of known fossils, stretching back six to seven million years and representing about two dozen species. Some were our ancestors and others distant cousins. "There have been many experiments in human evolution," Jungers says, "and all of them but us have ended in extinction." Our direct ancestors evolved in Africa some 200,000 years ago and started spreading out perhaps 120,000 years later. Remarkably, our modern human forebears shared parts of Europe and western Asia with the Neanderthal species as recently as 30,000 years ago, and they may have also overlapped with two other long-gone ancient humans, Homo floresiensis and Homo erectus, in Southeast Asia. "We were never alone on this planet until recently," Jungers says.
Darwin himself was confident that the deep past would be revealed. "It has often and confidently been asserted, that man's origin can never be known," he wrote in 1871. "But ignorance more frequently begets confidence than does knowledge: it is those who know little, and not those who know much, who so positively assert that this or that problem will never be solved by science." He also recalled, looking back on the shellacking he took for focusing on natural selection's role in evolution, that "the future must decide" whether "I have greatly overrated its importance." Well, the future has come down solidly on Darwin's side—despite everything he didn't know.
Asked about gaps in Darwin's knowledge, Francisco Ayala, a biologist at the University of California at Irvine, laughs. "That's easy," he says. "Darwin didn't know 99 percent of what we know." Which may sound bad, Ayala goes on, but "the 1 percent he did know was the most important part."
Thomas Hayden is the co-author of the 2008 book Sex and War: How Biology Explains Warfare and Terrorism and Offers a Path to a Safer World.