The diversity of life represents a balance between speciation and extinction.  The fossil record is our only significant source of data on patterns of extinction.  Given the sources of bias in the fossil record, one problem with estimating extinction rates is distinguishing "true" extinction from pseudoextinction.  Pseudoextinction "occurs" when one mistakes evolution occurring within populations for extinction. Consider a population at time t1.  By time t2 the same population has experienced some character evolution such that the morphology of the population at t1 and t2 are clearly different.  Now imagine we sampled fossil remains from the population at t1 and t2 - we might very well declare that the species at t1 went extint and was replaced by the species represented at t2, when in reality all that has occurred is evolution within a population (and no splitting or speciation at all).  This is pseudoextinction of the "species" from t1.  All of our discussions of pattens of extinction have as a premise, that pseudoextinction is not a major source of bias.

One interesting pattern of extinction is that the age a taxon is not a good predictor of the probability it will go extinct.  In other words, a "young" species is no likely to go extinct than an "old" species.  Thus, there is no support for the idea that older species are "better adapted" or more fit than younger species.

Two types of extinctions, mass extinctions and background extinctions, are distinguished in the literature.  Five periods of time are referred to as mass extinction.  As many as 80 to 90% of all species went extinct during each of these five extinctions.  These global, catastrophic events are distinct from background extinction, which has been ongoing throughout the history of life.  Diversity plots are one of the ways data on extinction and diversification are displayed.

These plots are meant to represent the temporal patterns of taxonomic diversity in different lineages.  Interpretation of such data are difficult, though they are sometimes used to attempt to determine if extinction in one lineage is possibly a consequence of diversification in another competing lineage.  If so, then the increase in diversification of one lineage occurs simultaneously with the decrease in diversity in another lineage.

The five mass extinctions occurred in the Ordovician (440 million years ago), the Devonian (370 MYA), the Permian (225 MYA), the Triassic (200 MYA) and the Cretaceous (65 MYA).


Large blue arrows point to five generally recognized mass extinctions, while other arrows point to other significant extinction events.
 

The best understood extinction is the KT or Cretaceous-Tertiary extinction of around 65 MYA, which caused the extinction of the dinosaurs.  The data strongly support the idea that a massive extraterrestrial object impacted the earth, setting off a global, ecological upheaval.  Three main pieces of evidence support this view.  First, deposits at the KT boundary are very iridium-rich.  This element is common in extraterrestrial objects but very rare on earth.  Second, there is a very large impact crater (118 km) of the correct age.  Third, there are abundant rocks of the correct age indicative of a high velocity impact.  Estimates put the asteroid that slammed into the Earth at about 10 km across.  The hypothesis is that the impact through massive amounts of particulates into the atmosphere, blocking much sunlight, impeding photosynthesis, and causing global cooling.  Speculation also has the impact triggering large scale volcanic activity which may have contributed to the ecological disaster.


Spike of the iridium at the KT boundary and impact crater associated with KT extinction


 
 
 
 
 
 
 
 
 
 

The particular species that survive a mass extinction do not do so because they were "better" than other species.  Natural selection cannot "prepare" a species for the future.  However, the traits that a species has, which were shaped by natural selection, could just by chance help the species make it through an environmental disaster.

Trilobites were an enormously successful group of organism, flourishing for about 200 million years before going extinct during the great Permian mass extinction.

As we should expect, species with wider geographic distributions are less likely to go extinct compared to species with narrower geographic distributions.  This is probably less of a factor in global catastrophes, than in more "normal" types of environment induced extinctions.

One of the consequences of differential extinction and speciation across lineages is that the prevalence of characters at any moment in time could partially reflect this process.  In other words, certain characters may be more common than others not because these characters are somehow "better" or make species "more highly adapted", but rather, because lineages having these characters tend to speciate more and go extinct less often, compared to other lineages.  This is known as species selection.  Note that such a phenomenon in no way negates the importance of normal adaptive evolution, but it does suggest the possibility of a different layer of causation vis-a-vis the distribution of characters across life forms.

Extinction is often thought of as leaving open previously occupied niches.  These "unoccupied niches" could stimulate adaptive radiations, in which new forms evolve to take advantage of new ecological opportunities.  One such example is thought to apply to the rise of mammals.  Mammals had existed for a long time, contemporaneously with dinosaurs.  However, mammals did not diversify or become very common until after the extinction of the dinosaurs.  One hyoothesis is that extinction of the dinosaurs opened niches into which mammals radiated.

An interesting question is whether there are long term trends in the history of life.  The question is difficult to answer, as the traits one might use and their quantification are problematic.  There has been some speculation that size has tended to increase during the history of life.  At some level this must be true, as life originated as single celled organisms.  It is also clearly true that the largest animal forms today are much larger than the largest animal forms that existed after the Cambrian explosion.  However, it is not clear that there has been a gradual trend of increasing size.  There is some speculation that escalating predator-prey interactions are a major cause of selection pressures resulting in the evolution of larger body size in animals.  There has also been some speculation that complexity of organisms has been increasing over time.  Again, in some trivial way this must be true.  However, it is entirely unclear whether the animals of today are generally more complex than the animals of 200 million years ago.  Part of the problem is that there is no generally accepted way of objectively quantifying complexity.
 
 

A brief discussion of the history of life, including some "key" moments:  Below is a timeline showing the emergence of particular life forms:



 

The Earth formed roughly 4.5 billion years ago.  The first fossil evidence of life is from about 3.5 billion years ago.  This suggests that life evolved relatively soon after conditions on the Earth became sufficiently benign to allow life (e.g., the existence of liquid water is probably necessary for life).  Some investigators suggest that the fact that life on Earth appears to have evolved fairly quickly supports the idea that life has evolved many times during the history of the universe.

We infer that the first life forms were prokaryotes.  Fossils interpreted as single-celled eukaryotes appear in the fossil record about 2 billion years ago.  Interestingly, there is no fossil evidence of multicellular life until about 550 million years ago.  One interpretation of these data is that the transition from single-celled to multicellular life is a "difficult" or unlikely one.  In fact, it is not at all clear that multicellularity would be strongly expected to evolve if the "tape of life" were run again, though there is some debate on this point.

Earliest known fossil life forms, and interpretations.  These are thought to represent strings of single cells,
much like modern blue-green algae (which are prokaryotes, not algae).
 
 

Left panels show modern prokaryotes, while right panels show phenotypically similar fossil forms.
 
 


Some of the first hypothesized eukaryotic life forms.

The Cambrian explosion refers to the appearance of all the extant animal phyla in the fossil record during a very brief time interval about 550 million years ago.  All currently known animal body plans appeared at this time.

Some of the animal body plans that are first seen in fossil deposits from the Cambrian.

There is considerable debate as to whether the animal phyla originated over a very short time period, as opposed to a longer history for these lineages that is not revealed by the fossil record.  However, regardless of the ages of the lineages, it is likely that the morphological diversity did appear over a relatively brief time period.

It has been difficult to explain why much of animal diversity appeared suddenly, and at this particular time.  Some speculate that an increase in oxygen concentration in the oceans provided opportunity for larger organisms with higher metabolic rates to evolve.  This could have set off predator-prey arms races that led to increasing complexity, diversity and size.

One of the best Cambrian fossil deposits is from the Burgess Shale.   Some of these strange animals appear to have left no modern descendants, while others are clearly ancestors of extant animals.


 

There is increasing evidence that animal forms evolved significantly early than the Cambrian, pointing to poor fossilization as the explanation for the sudden appearance of animals in the Cambrian deposits.  One piece of evidence are these Precambrian fossilized animal embryos.

 

Another piece of evidence is this 550 million year old fossil that appears to be a very primitive fish.


 

There are three major domains in the history of life.  These are the bacteria, the archaea, and the eucarya.  This is quite different from previous hypothesized organizations of life.  These three domains are well-supported by analysis of molecular data.  Archaea used to be aligned with other bacteria, however, recent data have revealed that these single celled forms are as distinct from other bacteria as we are from bacteria.  Archaea are often found in extreme environments, such as high temperature or high salt concentrations.  Thus, they are often referred to as extremophiles.

An interesting event in the history of eucarya is that organelles such as mitochondria and chloroplasts derive from an endosymiotic event.  That is, our mitochondrial are the descendants of a prokaryote that was engulfed by another cell type.  Presumably, the early relationship between these two cells was symbiotic - both cells enjoyed some advantage from the fact that one cell lived inside the other.  Eventually, mitochodria evolved and eventually lost the ability to exist independently of their hosts.  The fact that mitochondrial genes and genomes strongly resemble prokaryotic genomes in improtant ways certainly strongly supports this endosymbiont hypothesis.  Phylogenetic analysis of organelle genes also supports this idea.

Phylogeny showing that mitochondrial sequences cluster with bacterial genomes rather than with the nuclear genomes from the organisms
from which the mitochondria were isolated.

Human Evolution

Humans are great apes.

Humans are the only surviving species from  the hominid lineage.  The first hominids are from the genus Australopithecus.  They were small, bipedal primates that lived in Eastern or Southern Africa around 3-4 million years ago.

A more recent hominid genus, and a direct ancestor of Homo sapiens,  is Homo erectus, which lived about a million years ago.  The most obvious trend in human evolution is that our relative brain size increased dramatically over the last three million years.  Here are some data that document this trend.

A new and very interesting fact from recent fossil discoveries is that the genus Homo was quite diverse at some time in the fairly recent past.  In fact, it now appears that several early Homo species coexisted in Africa.

The very recent history of modern humans is hotly debated.  The preponderance of data, however, support the idea that modern humans, Homo sapiens, evolved in Africa, and then spread throughout Eurasia, probably outcompeting and replacing earlier humans.  African populations of humans are more genetically variable than other populations, and European genetic diversity is a subset of the variation present in Africa - both types of data support the African ancestry of our species.