The Ascent of Mind: Ice Age Climates and the Evolution of Intelligence, by William H. Calvin, Bantam Books 1991, is an interesting set of hypotheses which try to explain the unusual increase in human brain sizes over the past 2.5 million years.
The obvious explanation (that increased intelligence associated with increased brain size increased their fitness) runs into many problems, such as why the change happened so suddenly after millions of years of little change, and why no other apes showed similar changes. Calvin's explanation says that the repeated boom and bust cycles of population growth associated with the advance and retreat of ice caps provided the opportunity for rapid fluctuations in brain size. In the boom cycle of an ice age, newly uncovered land provided an opportunity for rapid population growth, which favors early puberty, which as an accidental byproduct creates a higher brain-to-body ratio.
Normally, as soon as this boom ended, the brain-to-body ratio would slip back to the norm, as happened with other mammals. Calvin speculates that a crude weapon (known as a killer frisbee or hand ax) provided the rewards to larger brains because it was very useful at killing game when thrown into the middle of a herd with the right timing. The need for more precise firing than individual neurons can handle created an advantage to averaging the firings of large numbers of neurons available only in larger brains.
The ice age cycle is also essential to the spread of these traits from the fringes of the species to a dominant position. Normally with a population as large as Homo sapiens apparently had over this period, evolutionary changes take a very long time to spread to the whole population unless failure to adopt the new traits is disastrous. The large increase in the fringe population during the boom phases of the ice ages created a rapid increase in the ratio of new to old traits, and as long as the new population could substantially migrate back to warmer climates when the ice returned, this increase could have been maintained.
The book is somewhat diminished by a substantial number of irrelevant anecdotes, mostly moderately good stories about geology, but also some annoying claims about the need to limit population growth until we can understand its effects on the climate.