Introduction

Earth's evolutionary divergence from the neighboring planets of the solar system, beginning with the stabilization of liquid water, culminates in the appearance of sentient organisms sometime within the past 1 million to 2 million years. The fossil record is abundant in its yield of creatures intermediate in form and function between the great apes and modern humans; new discoveries seem to be made with increasing pace. But hidden between and among the fossil finds are the details of how and why we came to be. Even as we acknowledge our common origins with the life around us, the singular results of sentience – art, writing, technology, civilization – are surprising and enigmatic.

The story of human origins is not simple, and changes with every new fossil find. Therefore, this chapter attempts only a sketch of the evidence and the lines of thought current in today's anthropological research. It begins with a broad view of the climatological stage on which these events took place. It ends with a focus on the closing act of human evolution, the coexistence of modern humans with a similar but separate sentient species in Europe and the Middle East – the Neanderthals.

Pleistocene setting

The earliest fossils along the lineage toward humanity exist in the Pliocene epoch, prior to the Pleistocene, during a time of relative climate stability. The pace of human evolution picks up in the Pleistocene, and species close enough in form to us to warrant assignment to the genus Homo (Latin, man in the sense of humans) appear close to, but perhaps slightly before, the time when climate shifted into an ice-age pattern of glacial and interglacial episodes.