The story of life since the great meteor impact, 66 million years ago, is basically a story of global cooling. Many factors contributed to this cooling: mountain folding, the configuration of continents with attendant changes in sea currents, biological events, the closure of the warm, shallow Tethys Sea, etc. The closure of the Panama Gap made North and South America a continuous land mass (leading to a major extinction of South American fauna), and blocked the warmer Pacific waters from entering the colder Atlantic (sugaring the pill by getting the Gulf Stream started).

Regardless of causes, two features of the last 66 million years seem clear. We live in a "post-meteoritic" age, and we live in an age when continents are again drawing closer. We have never had dinosaurs to deal with (the impact killed them all), nor the lush and predatory tropical climates in which they thrived. Our kind (primates, mammals) grew up in a harsher world, slowly cooling and drying, culminating in the Quaternary Ice Age that has lasted, at the time of writing, approximately 2.58 million years and has seen at least 3-4 dozen glaciations of increasing severity come and go, covering larger and larger areas of the Earth for longer and longer periods of time.

Note that there have been at least two similar periods earlier in the Phanerozoic (see Timeline 4):

(i) The Andean-Saharan Ice Age (about 450 mill BP), which occurred when life was still confined to the sea and appears to have coincided with (and probably to have caused) the great Silurian-Ordovician Extinction.

(ii) The Karoo Ice Age (around 300 mill BP), which lasted nearly 50 million years and coincided with the period of maximum concentration of the Supercontinent Pangea (which brought colder weather). This Ice Age was also followed by an extinction, the great Permian Extinction, and though there seems to be no causal relationship between the two events, it is probable that the extinction was worsened by the already stressed condition of the ecosystems it affected.

It seems worthwhile to contemplate these previous Phanerozoic ice ages in the light of our own situation. If our Ice Age turns out to be as long as the Karoo, we may be at the start of a 20-30 million year cold period, which may definitely become more severe than during the last glaciation. Given the marked weakening of global ecosystems during the last few hundred years, it would not be surprising if the next glaciation becomes a particularly severe one, or even the occasion of a new extinction, of Andean-Saharan type (caused by ice age) if not proportions.

This is the age of giant, flightless predator birds, and, somewhat later, giant mammals. But it is also the age in which many animals known to us originated, including, ultimately, ourselves. This dry and hardy age, is also the age in which grass originates and rapidly spreads, replacing trees in many areas, supplanting ferns as the major food for grazers, and killing all species unable to digest the tougher grasses.
 
Among primates, the anthropoid lineage evolves and quickly differentiates (c 31-38 mill BP) into monkeys and apes. Apes, to which humans belong, have menstrual cycles, use tools and build shelters. Most were, at the time, tree-dwellers and diversified into a wide variety of species. Only around four million years ago do the first human-like apes appear - a small, fast and resourceful creature known as australopithecus.