Before going further, it would be helpful to present a definition of what evolution is:
Further definitions from the same source are also germane:
The aim of the project presented in this website is to examine the fossil record of all major groups of animals and plants and their ancestors. There are several ways of finding information on any group of interest:
In addition to the text and illustrations directly available in the pages of this website, detailed fossil information is provided in data files that can be downloaded from the site. Note also that clicking on highlighted words (e.g. glossary) will open a glossary containing a brief definition of terms used.
To date, only part of the vertebrates has been completed; more will be added as the work is finished.
- Evolution consists of changes in the heritable traits of a population of organisms as successive generations replace one another. It is populations of organisms that evolve, not individual organisms (US National Academy of Sciences, 2018).
Further definitions from the same source are also germane:
- Macroevolution: Large-scale evolution occurring over geologic time that results in the formation of new species and broader taxonomic groups.
- Microevolution: Changes in the traits of a group of organisms within a species that do not result in a new species.
The aim of the project presented in this website is to examine the fossil record of all major groups of animals and plants and their ancestors. There are several ways of finding information on any group of interest:
- In the menu bar above (under "Evolution of Life")
- From highlighted links in pages reached through the "Evolution of Life" tab above
- Using the search box at the top of every page.
In addition to the text and illustrations directly available in the pages of this website, detailed fossil information is provided in data files that can be downloaded from the site. Note also that clicking on highlighted words (e.g. glossary) will open a glossary containing a brief definition of terms used.
To date, only part of the vertebrates has been completed; more will be added as the work is finished.
Approach
This website tracks the process of macroevolution as seen in the fossil record. In so doing, it focuses on the major transitions that took place over geological time. At this point we need to consider the question of “transitional fossils”.
The idea of transitional fossils is much misunderstood. The common concept of evolution is that of a single chain representing a linear process; this has given rise to the idea of “missing links” or “transitional fossils”, the absence of which could call into question the validity of the theory of evolution. The chain concept has also engendered objections such as “If humans descended from monkeys, why are there still monkeys?”
These objections are based on a misunderstanding of the evolutionary process. Rather than a single “chain of life”, evolution has generated a tree with a huge number of branches. Thus the evolutionary concept is that of a “tree of life’ in which ancestors give rise to descendants and those descendants in turn generate further descendants, just as in a family tree. According to this view, transitional or intermediate forms only exist as extinct branches of the tree:
The idea of transitional fossils is much misunderstood. The common concept of evolution is that of a single chain representing a linear process; this has given rise to the idea of “missing links” or “transitional fossils”, the absence of which could call into question the validity of the theory of evolution. The chain concept has also engendered objections such as “If humans descended from monkeys, why are there still monkeys?”
These objections are based on a misunderstanding of the evolutionary process. Rather than a single “chain of life”, evolution has generated a tree with a huge number of branches. Thus the evolutionary concept is that of a “tree of life’ in which ancestors give rise to descendants and those descendants in turn generate further descendants, just as in a family tree. According to this view, transitional or intermediate forms only exist as extinct branches of the tree:
Ancestors represent nodes of the tree and are shown as black dots. However, we cannot identify ancestors in the fossil record; we can only find the fossils that are descended from them. Fossils a and b are extinct descendants of ancestors A and B respectively, while extant form c is a living descendant of ancestor C. Ancestor B is transitional between ancestor A and ancestor C, but its closest representation in the fossil record is fossil b. Thus fossil b is only an approximate representation of the intermediate ancestor B. If we apply the above diagram to relationships within the archosaurs (crocodylians, dinosaurs and birds), extant form c might represent birds, fossil b could represent dinosaurs, and fossil a could represent extinct crocodylians. However, dinosaurs are only transitional between crocodylians and birds in the sense that they have an ancestor that is intermediate between the ancestors of the crocodylians and the birds. All three groups share a common ancestor, that of all archosaurs.
The tree configuration illustrated above is further developed in the concept of crown and stem groups, ideas critical to an understanding of how evolution is represented in the fossil record. A crown group is the smallest monophyletic group, or clade, to contain the last common ancestor (LCA) of all extant members, and all of that ancestor’s descendants. A stem group, on the other hand, comprises those fossils which are closer to the crown group than to any other extant clade, but do not fall within the crown group. We could say that in the diagram above, ancestor C represents the crown node, extant form c belongs to the crown group, and fossil b represents the stem group corresponding to that crown group. But the concept is depicted more fully in the figure below:
The tree configuration illustrated above is further developed in the concept of crown and stem groups, ideas critical to an understanding of how evolution is represented in the fossil record. A crown group is the smallest monophyletic group, or clade, to contain the last common ancestor (LCA) of all extant members, and all of that ancestor’s descendants. A stem group, on the other hand, comprises those fossils which are closer to the crown group than to any other extant clade, but do not fall within the crown group. We could say that in the diagram above, ancestor C represents the crown node, extant form c belongs to the crown group, and fossil b represents the stem group corresponding to that crown group. But the concept is depicted more fully in the figure below:
The stem group and crown group together represent the total group. Crown and stem groups are organized hierarchically, such that a total group at one level of hierarchy contains total groups at the next level. Thus in the figure above, crown group A bifurcates into stem/crown group A1 and stem/crown group A2.
Another important concept is that of the ghost lineage, illustrated in the figure below:
Another important concept is that of the ghost lineage, illustrated in the figure below:
Consider taxon A, which is interpreted to have bifurcated into taxa B and C. However, the fossils so far found of taxa B and C do not share the same range of geological time. Given that taxa B and C, which arose from a single bifurcation event, must have appeared at the same time, the delay in appearance of taxon C, relative to that of taxon B, represents the lineage of taxon C that has not yet been found in the fossil record. This is the ghost lineage of taxon C.
The approach followed in this website is to track the progress of evolution by documenting the appearance in the fossil record of the earliest representatives of successive crown groups and their associated stem groups. The time of first appearance is expressed as a range of uncertainty, and is based on the latest available information on the age of the rock unit in which the fossil in question appears. In some cases, this age differs from that presented in the original publication describing the fossil. The supporting data for this documentation is given in a PDF file accessible through the button below. In order to do this, a phylogenetic tree must be assumed as a starting point. This website uses published trees for which there are no unresolved disagreements in the literature. These published trees are based on molecular (DNA) evidence, morphological data, or a combination of the two (Hillis, 1987; Hermsen and Hendricks, 2008).
The approach followed in this website is to track the progress of evolution by documenting the appearance in the fossil record of the earliest representatives of successive crown groups and their associated stem groups. The time of first appearance is expressed as a range of uncertainty, and is based on the latest available information on the age of the rock unit in which the fossil in question appears. In some cases, this age differs from that presented in the original publication describing the fossil. The supporting data for this documentation is given in a PDF file accessible through the button below. In order to do this, a phylogenetic tree must be assumed as a starting point. This website uses published trees for which there are no unresolved disagreements in the literature. These published trees are based on molecular (DNA) evidence, morphological data, or a combination of the two (Hillis, 1987; Hermsen and Hendricks, 2008).
Assumptions
- The earth has an age of between 4 and 5 billion (using "billion" as 10 to the power of 9) years, as indicated by standard geological time scales such as the International Chronostratigraphic Chart produced by the International Commission on Stratigraphy. Version 2015/01 of this chart is used as the time scale throughout this website.
- Evolutionary relationships for which there is no unresolved disagreement in the literature represent the best available scientific consensus. Note that the website contains no independent assessment of evolutionary relationships between or within taxa.
Additional information
Files containing detailed fossil data can be found here or in the site navigation menu under "Data" .
References cited in the website are given here or in the site navigation menu under "Evolution of Life" .
Photo credit
Header: By Wolfgang Sauber (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
Header: By Wolfgang Sauber (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons