Cenozoic life




The basis for the information

Explanatory comments

Tertiary Period



 Life recovers from extinction

Fossil record of the Paleocene and Eocene give us good documentation of recovery from the mass extinction


Mammals and birds become dominant animals in land ecosystems

Fossil record


Earth cooling, Antarctic ice cap grows, at end Ice Ages in Northern Hemisphere begins

Cooling is suggested based on oxygen isotopes and other measures


Proconsul and similar early hominids in Micoene

Proconsul is the best known primate hominoid from about 25 million years ago (the early Miocene) in Africa

Proconsul still had many ape-like features, e.g., in the ankles, feet, and teeth, but had an opposable thumb, shoulder, and comparatively large brain like later hominids

Bipedal australopithecine hominids appear in Pliocene

Australopithecus and relatives found from about 5 million years ago in Africa

Australopithecus seems to be an offshoot of the hominid tree, rather than a direct ancestor to Homo

Quaternary Period



Ice sheets advance and retreat over North America about 50 times

We have a particularly detailed record from the sediments of deep-sea cores; 18O stable isotopes give us information about sea level


Glaciers carve the land and influence the position of animal populations

Glaciers leave behind characteristic sediments that show how far they traveled. However, they also erase the records of previous glaciers.


At end of Quaternary, after last glacial maximum, people start to populate the globe with advanced spearpoint technology

We find human remains and artefacts around the world by about 15,000 years ago

Homo sapiens seems to have arisen about 200,000 years ago in Africa; “modern” H. sapiens with cultural sophistication first appear in Europe and in South Africa about 50,000 years ago

Large mammals start going extinct from around globe about the time that humans migrate into new areas, suggesting that humans are responsible for the extinction of these mammals

Many fossil remains of large animals, including esp mastodons, have characteristics that suggest human butchery, such as distinctive cut marks and breakage along certain points

Spearpoints are occasionally found directly with bones







Biological evolution refers to essentially permanent change in populations of organisms or in species. The concept of evolution is a fairly old one, dating at least to the late 18th century, but in the mid to late 1800s evolution was shown to provide an explanation for a wide variety of observations that were being made by naturalists at the time. Darwin was the best known both because he compiled an extensive list of evidence beyond anyone else at the time, and he also provided a potential mechanism for evolution known as “natural selection.” The scientific community continues to work on the means by which evolution occurs, but the broad consensus has long been that evolution is the best explanation for the patterns we see in the diversity of life. Understanding mechanisms of evolution have a number of practical applications, not least of which is controlling organisms with short generation times such as disease-causing bacteria, since their evolution is responsible for drug-resistance and the formation of new strains. 


Patterns explained by evolution


Fossil record: Organisms that are anatomically similar are also found close together in the rock record, and progressively more complex organisms occur toward the upper (younger) portion of the rock record. No inexplicable deviations have been found in 200 years of fossil collecting and countless millions of specimens, many of which have been depended upon for applied problem solving in high-stakes fields such as petroleum geology.


Comparative anatomy: Organisms share similarities in structure that make sense in light of their using what they have to adapt to local environments. Organisms frequently maintain aspects of their anatomy that were relevant in their evolutionary past, such as vestigial hind limbs in whales.


Biogeography: Taxa of organisms that live near each other tend to share many anatomical similarities, even when they look rather different; organisms often diversify to fill local niches that are available. For example, some marsupials of Australia look superficially more similar to animals in other parts of the world with whom they share similar adaptations, but their anatomy and development show that in fact all marsupials are more closely related to each other than any are to mammals outside the marsupials.


Nested hierarchy of taxonomy:  If any animal can potentially have any characteristics that suit it more adaptation’s sake, then it will not be possible to organize taxa into nested groups the sort of nested hierarchy of taxonomy (Linnean classification) that we have been using for over 200 years. The more closely related animals are, the more similar they are anatomically and the more that will be reflected in being in the same taxon. If any animal can potentially have any characteristics that suit it more adaptation’s sake, then organisms are less likely to show nested sets of similarity.


Patterns of genetic similarity: One can look at genetic material in many of the same ways that one looks at the shape of organisms. Some genetic material codes for enzymes that are critical. But sometimes if the genetic code is changed in certain respects there is no resultant effect. This part of the genetic code changes through occasional mutation at a relatively constant rate. If genetic similarity in this material matches with patterns of biogeography, comparative anatomy, and nested hierarchy of taxonomy,


The origin of species diversity


Diversity: In our lessons and in these notes, “diversity” refers to the number of kinds of living organisms, in particular the number of species of animals.  In many cases we count the number of families because it is more practicle, and we assume that the more families there are, the more species there are.



Number of taxa = number of new taxa (speciation) – number of extinct taxa + number of taxa you started with (for any individual area: +/- migration)


Species are “real” – they are populations of organisms that interbreed and produce fertile offspring; we often have to estimate species by differences in shape, if we can’t watch this process; genera through phyla are groupings of related species, but otherwise are not biologically real in the same way


Speciation: A new species develops when one part of a species becomes so different than the rest of the species that it no longer interbreeds with it; a typical way for speciation to happen would be for a populartion of a species to become genetically isolated and adapt to their local habitat, to the point that they no longer mate with the other populations, even if given the opportunity; this is speciation


Some views hold that a lot of evolution occurs as whole species transform, due to widespread environmental change; a traditional view of the horse showed a “ladder” of evolution from small to large horses adapting to grassland habitats


Some if not most of evolution occurs at times of speciation, rather than the transformation of whole species; the story of the horse is that lots of speciation events occurred, some creating grassland horses, some creating other kinds of horses – a “bush” rather than a ladder; today the only horses are large grassland horses, but their evolution has occurred over a series of speciation events and greater survival of large than small species.