Sometimes the maturity of a field of science can be measured by the heft of its ambition in the face of the next daunting unknown, the mystery yet to be cracked.
Neurobiology probes the circuitry of the brain for the secrets of behaviors and thoughts that make humans human. High-energy physics seeks and may be on the verge of finding the so-called God particle, the Higgs boson thought to endow elementary particles with their mass. Cosmology is confounded by dark matter and dark energy, the pervasive but unidentified stuff that shapes the universe and accelerates its expansion.
In the study of human origins, paleoanthropology stares in frustration back to a dark age from three million to less than two million years ago. The missing mass in this case is the unfound fossils to document just when and under what circumstances our own genus Homo emerged.
Good long article.
UPDATE: Related.