I've been doing a little bit of research, based on the observation that humans are ice age creatures, even though ice ages have made up only 5% or so of our current era (roughly the past half billion years or so). So today's topic is, "What was the earth like during the much warmer climatic periods during which humans and our precursors hadn't yet evolved?" An implication of this discussion is that, due to human-initiated massive carbon release, we might be headed back to such conditions sooner rather than later (that is, in a few hundred years, vs. several million years).
The scientist I have so far identified who seems most interested in this question is Dr. Rick Potts at the Smithsonian Institution. The following is an abstract for one of his journal articles.
THE RICK POTTS HYPOTHESIS
Variability selection (abbreviated as VS) is a process considered to link adaptive change to large degrees of environment variability. Its application to hominid evolution is based, in part, on the pronounced rise in environmental remodeling that took place over the past several million years. The VS hypothesis differs from prior views of hominid evolution, which stress the consistent selective effects associated with specific habitats or directional trends (e.g., woodland, savanna expansion, cooling). According to the VS hypothesis, wide fluctuations over time created a growing disparity in adaptive conditions. Inconsistency in selection eventually caused habitat-specific adaptations to be replaced by structures and behaviors responsive to complex environmental change. Key hominid adaptations, in fact, emerged during times of heightened variability. Early bipedality, encephalized brains, and complex human sociality appear to signify a sequence of VS adaptations—i.e., a ratcheting up of versatility and responsiveness to novel environments experienced over the past 6 million years. The adaptive results of VS cannot be extrapolated from selection within a single environmental shift or relatively stable habitat. If some complex traits indeed require disparities in adaptive setting (and relative fitness) in order to evolve, the VS idea counters the prevailing view that adaptive change necessitates long-term, directional consistency in selection. © 1998 Wiley-Liss, Inc.