Through Time and Across Traits: Evaluating Explanations of Human Evolution

We argue that current explanatory practice misses two situations that can arise in any lineage, not only ours. The first is dynamic: traits continue changing in one direction when a lineage's own niche construction keeps enlarging the problem it faces. The second is unifying: many traits change together and may require one explanation rather than several. Both cases bear on Tinbergen's four questions, the test of explanatory completeness, and both require extending his function question: what is a trait for? An answer that names a problem of fixed size cannot explain continued change. We call a problem of rising magnitude a gradient, and one that serves many traits a unifying gradient. We set out five questions that any complete gradient explanation must answer, map the major hypotheses of human evolution onto this scheme, and use our Prey-Size Decline hypothesis as a worked example. The proposed framework does not depend on that example.