Hominin evolution took place during a time of great global climate change. Drilled-out deep sea and glacier cores hold historical records of the Earth's climate. Each layer of ice that formed, and each layer of sea floor that was deposited, contains a snapshot of the climatic conditions during its creation. Those conditions are told by the amount of certain chemical compounds in the layers. Times of cooling (glacials) or warming (interglacials) are indicated by ratios of oxygen isotopes differentially taken up by microscopic, one-celled organisms (foraminifera or "forams") that are fossilized in the layers.
Some patterns suggest that extinction rather than speciation events are more likely to be correlated with climate change. However, extinction resulting from such changes can open up niches for new organisms to fill. The extinction of the dinosaurs at the Cretaceous-Tertiary boundary was gradually followed by the enlargement of mammals that eventually filled the empty small- and large-bodied dinosaur niches.
The evolution of the earliest primates immediately predated the rapid global cooling that began in the Eocene. Another long-term cooling and drying trend coincided with the origin ofhominins between 8 and 5 Mya. Oscillations in global climate occurred during hominin evolution and these oscillations probably affected the outcome of human evolution. Starting in the late Pliocene around 2.8 Mya Africa became cooler and dryer, which likely converted forests to grasslands. East African vegetation transitioned from closed forested canopies to open, arid savannahs with reduced and seasonal precipitation.
Then continuing through the lower Pleistocene (1.8 Mya-900 Kya) the world climate began to cool more rapidly (Figure 3.4). The middle Pleistocene (900-125 Kya) was characterized by cold glacial periods, for 100,000-year cycles, interspersed with warmer interglacial periods. And this was followed by the "Ice Ages" of the Upper Pleistocene (125-12 Kya), which were also very cool glacial periods that were broken up by warm periods. During the colder periods, water would have been locked up and frozen in glaciers, causing sea levels to lower. Lower sea levels probably had a great impact on hominin mobility since isolated islands or distant continents could become attached to the mainland by dry land. Then when the ice caps melted in a warm interglacial phase, those regions could become islands again and isolate the species living on them.
Fossils of animals and plants at a hominin site are useful for reconstructing the paleoenvironment in ways that differ from the tiny forams in sea cores. Based on what we know about the diverse ecology of living bovid species (antelopes and the like) the types of fossil bovids at a site can indicate whether it was open grassland or more closed woodland. Fossil monkeys are also reliable indicators of environment: if they have arboreal adaptations in the skeleton or teeth that are associated with leaf-eating (like modern colobines), then the environment was probably forested. If they have terrestrial adaptations like baboons then the hominin site may be much more open, arid, and savannah-like. Often crocodile teeth are scattered everywhere at sites near ancient watering holes or ancient riverbeds. The more animals a paleontologist can use to reconstruct the paleoenvironment, the stronger the science.
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