Prior to the onset of our current ice age, Earth was in a greenhouse climate. Greenhouse systems are distinct from our familiar icehouse era in the effects of seasons upon the environment. In our icehouse age, seasonal progression across most of the planet transitions from hot/wet to cold/dry. The hot/wet summer-spring months are the most temperate and life supporting. Selective pressure is exerted on life during the cold/dry fall-winter months. The relationship of the seasons to life is inverted in a greenhouse climate. During the hottest months, moisture within tropical biomes begins to evaporate. Hot months are often absent of rainfall producing long droughts during the summer. Hot conditions place extreme selective pressure upon life systems. The return of moisture is associated with the cool period - Earth's winter. The characteristic yearly change of the seasons isn't associated with the shift from woodlands to tundra that takes place throughout most of the northern hemisphere in the modern era, but instead periodically caused a shift from rainforest to desert climate.

Plant and animal species flourishing across the planet during the greenhouse period were primarily adapted to the tropical and desert climate. These species are distinguished by their lack of genetic adaptation to cold weather. Selective factors in evolution within a greenhouse environment produce a different ecological system as its outcome. Generally, reptiles and insects have an evolutionary advantage in hot and dry environments - their bodies make them nearly indifferent to temperatures which would kill warm-blooded mammals. Much like rainforest biomes today, insects and reptiles excel within jungle ecosystems while mammals must be highly specialized and adaptive. In a greenhouse climate, the yearly change in seasons from tropical to desert would have little effect on reptile or insect species, who could easily endure hot periods without much loss of life. Recall however, that many insects are subject to seasonal displacement in the present climate.

Plant species of the prevoius greenhouse period possessed anatomical adaptations which made them uniquely suited to the inversion of seasons. Evergreens are a class of plant which don't lose their foliage in response to seasonal changes. There are a variety of evergreen species which have adapted to cold and warm climate alike. Prior to the onset of the Cenozoic, these species dominated the landscape. Deciduous plants are their counterpart - weilding the ability to shed foliage in response to seasonal and environmental cues. The trigger for shedding to occur in deciduous planet species isn't a change in temperature or season, as is often mistakenly assumed, but is instead correlated to a lack of rainfall. Plants abandon their leaves to preserve resources, opting instead to pull nutrition exclusively from the soil. The deciduous mechanism can successfully operate in hot and cold climate alike. Deciduous species in hot climates (greenhouse) lose their leaves with hot months in response to drought, instead of cold months. Prior to icehouse conditions there wasn't much evolutionary advantage in reactive shedding. Across most of the globe seasonal change was moderate during the greenhouse period, offering little benefit over evergreen species competing in the same biosphere.

When Earth transitioned into the Cenozoic selective pressure upon all species was reversed. Life systems across the planet whose characteristics had for millenia evolved within a moderate-warm climate were upended. Cold-weather pressure became the new standard. Instead of the hottest regions being inhospitable, many became an oasis from icehouse cooling. Most importantly the seasons began to exert real selective pressure on life. One of the first consequences of the transition was the spread of deciduous plant species. According to some researchers, this was a pivotal point in the history of life on Earth.

In the previous era, the dynamics governing evolution across the globe followed the same basic pattern. Most life supporting regions were tropical. The most inhospitable areas were deserts. Seasonal change occurred from tropical (wet) to desert (dry). Thus, the most adaptive life forms were insects/reptiles. When environmental pressure increased, evolution continuously favored these species. Their populations boomed throughout the greenhouse period. Mammal species didn't have much of a "niche". With the onset of the ice age, life ecosystems became cooler everywhere. The most inhospitable areas became dry tundras instead of dry desert. Seasonal change began to resemble the modern period. During the early cenozoic, deciduous plant species began outcompeting evergreen species. Seasonal adaptation became more practical. Tropical plant species that had evolved during the greenhouse period had adapted to excessive heat, not cold. Consequently, most evergreen species were unable to endure cold periods. The success of the deciduous species meant that cold regions could sustain animal life, instead of becoming barren tundra under pressure. Deciduous species facilitated the spread of mammals who could endure cold temperature when warm weather animals could not.

In the new climate, mammals fared better than warm weather species of the past. Warm weather species became restricted to a narrow territory in the most temperate zones. Active stadial periods of glaciation were cataclysmic for warm weather species. Mammals had an advantage - they were capable of occupying territory inaccessible to their competitors. With the rise of deciduous plants, woodlands and tundra which experienced regular freezing and thawing due to the seasons could sustain mammal life. Groups of mammals who were driven from highly populated tropical regions due to extreme competition could now find refuge in cooler climates. These isolated cold weather regions allowed mammals to evolve within a "buffer zone" free from selective pressure exerted by other species. Novel mammal species began appear - horses are one noteworthy example. In time, the Cenozoic produced most of the mammals familiar to us today, including humans.

Throughout the Cenozoic, the cyclical transition from stadial (glaciation) to interstadial (deglaciation) has occured at varying intervals. The pace of the stadial cycles seems to directly mediate the nature of life on Earth for all species. Stadial acceleration means mass extinction, limited resources, genetic isolation, and climate shifts occuring at very short intervals. Acceleration makes it difficult for complex environments to manifest on Earth. The entire planet reverts to one of two extremes in icehouse conditions; either frozen tundra, or desert on most continents. Habitable ecosystems capable of supporting large populations are uncommon. Somewhat paradoxically, seasonal influence on life is dampened as the effects of active glaciation across the planet take precedence over minor fluctuations.

Life forms were specially adapted to the extreme cold and heat. There were scare resources - brutal competition taking place between apex predators and pack animals. Mass migration was a seeminly continuous phenomenon in most land dwelling animals as herds of herbivores relying on plant life for sustenance were often cut off from their food supply abruptly. Predators rely on herd animals for hunting - as such they migrate with the herd. Throughout the cenozoic, the geography of Earth was transforming due to the influence of glaciation. A displaced continent meant that life ecosystems which were previously in a state of relative homeostasis underwent rapid transformation. Populations relying on a moderate climate may be confined to an extremely limited territory - one unable to support pre-stadial populations. Thus the most delicate species were prone to mass-die of or extinction. The Cenozoic era and ice ages generally, are the ecological precedent for several important selective axioms in life evolution. Namely, species most suited to adaptation tend to survive, while the most novel and exotic die off. The Darwinian process in the early Cenozoic (34 MYE) operated at a pace which would quickly eradicate many species which thrive on Earth today.

At the beginning of the Cenozoic, Milankovitch cycles were operating at around 40,000 year intervals around the Earth. As mammals evolved, they did so within an unstable ecosystem. This made it difficult for more "sensitive" organisms to gain a foothold as populations were subject to regular extinction events. Around 2.58 million years ago, the pace of the ice age decreased. Instead of cycling at 40,000 year intervals, stadials shifted to 100,000 year events. The shift marked the beginning of the Quaternary period within the Cenozoic age. With it, the first indications of genus Homo - modern man's most recent ancestor.

The Quaternary period is further subdivided into the Pleistocene and Holocene. The vast majority of the Quaternary falls within the Pleistocene, which stretches from 2.58 MYE until 11.7 MYE. During this extended period, the direct ancestors of modern man began appearing around the world. From an extremely early period in in the Pleistocene, members of genus Homo gained several advantages over the climate which had never before existed on Earth. The discovery of fire and clothing allowed early man to endure when all other species had been driven away. This alone may account for the dramatic rise in populatiotn of our ancestors across the Earth since the start of the Quaternary period. Some evidence suggests that large groups of early human ancestors had adapted even to the most harsh regions of glaciation during stadial periods. The Cenozoic ice age no longer commanded the power of extinction over life on Earth.