Fossil Evidence
The fossil record provides snapshots of the past that, when assembled, illustrate a panorama
of evolutionary change over the past four billion years. The picture may be smudged in places
and may have bits missing, but fossil evidence clearly shows that life is old and has changed
over time.
Early fossil discoveries
In the 17th century, Nicholas Steno shook the world of science, noting the similarity
between shark teeth and the rocks commonly known as "tongue stones". This was our first
understanding that fossils were a record of past life.
Two centuries later, Mary Ann Mantell picked up a tooth, which her husband Gideon thought to
be of a large iguana, but it turned out to be the tooth of a dinosaur, Iguanodon. This discovery
sent the powerful message that many fossils represented forms of life that are no longer with
us today.
Additional clues from fossils
Today we may take fossils for granted, but we continue to learn from them. Each new fossil
contains additional clues that increase our understanding of life’s history and help us to
answer questions about their evolutionary story. Examples include:
Indication of interactions
This ammonite fossil (see left) shows punctures that some scientists have interpreted as the
bite mark of a mosasaur, a type of predatory marine reptile that lived at the same time as the
ammonite. Damage to the ammonite has been correlated to the shapes and capabilities of
mosasaur teeth and jaws. Others have argued that the holes were created by limpets that
attached to the ammonite. Researchers examine ammonite fossils, as well as mosasaur fossils
and the behaviors of limpets, in order to explore these hypotheses.
Clues at the cellular level
Fossils can tell us about growth patterns in ancient animals. This is a cross-section through a
sub-adult thigh bone of the duckbill dinosaur Maiasaura. The white spaces show that there
were lots of blood vessels running through the bone, which indicates that it was a fast-growing
bone. The black wavy horizontal line in mid-picture is a growth line, reflecting a seasonal
pause in the animal’s growth.
Nicholas Steno's anatomical drawing of an extant shark (left) and a fossil shark tooth (right). Steno made
the leap and declared that the fossil teeth indeed came from the mouths of once-living sharks.
http://evolution.berkeley.edu/evosite/lines/Ifossil_ev.shtml
Anatomical Evidence
Individual organisms contain, within their bodies, abundant evidence of their histories. The
existence of these features is best explained by evolution.
Several animals, including pigs, cattle, deer, and dogs have reduced, nonfunctional digits,
referred to as dewclaws. The foot of the pig has lost digit 1 completely, digits 2 and 5 have
been greatly reduced, and only digits 3 and 4 support the body. Evolution best explains such
vestigial features. They are the remnants of ancestors with a larger number of functional
digits.
People (and apes) have chests that are broader than they are deep, with the shoulder blades
flat in back. This is because we, like apes, are descended from an ancestor who was able to
suspend itself using the upper limbs.
On the other hand, monkeys and other quadrupeds have a different form of locomotion.
Quadrupeds have narrow, deep chests with shoulder blades on the sides.
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Hoatzin chicks have claws on their wings, as do some chickens and ostriches. This
reflects the fact that bird ancestors had clawed hands.
Organisms that are closely related to one another share many anatomical similarities.
Sometimes the similarities are conspicuous, as between crocodiles and alligators, but in other
cases considerable study is needed for a full appreciation of relationships.
Modification of the tetrapod skeleton
Whales and hummingbirds have tetrapod skeletons inherited from a common ancestor. Their
bodies have been modified and parts have been lost through natural selection, resulting in
adaptation to their respective lifestyles over millions of years. On the surface, these animals
look very different, but the relationship between them is easy to demonstrate. Except for
those bones that have been lost over time, nearly every bone in each corresponds to an
equivalent bone in the other.
Embryology Evidence
In embryology, the developing fetus is studied, and similarities with other organisms are
observed. For example, annelids and mollusks are very dissimilar as adults. If, however, the
embryo of a ragworm and a whelk are studied, one sees that for much of their development
they are remarkably similar. Even the larvae of these two species are very much alike. This
suggests that they both belong to a common ancestor. It is not, however, true that a
developing organism replays its evolutionary stages as an embryo. There are some similarities
with the more conserved regions, but embryonic development is subjected to evolutionary
pressures as much as other areas of the life cycle.
The analysis of developmental stages of various related species, particularly involving
reproduction suggests common descent and supports evolution. Sexual reproduction in both
apes and humans, for example, is very similar. Molecular biology also produces evidences
supporting evolution. Organisms such as fruit flies have similar gene sequences that are active
in specific times during development and these sequences are very similar to sequences in
mice and even humans that are activated in similar ways during development.
Even in cell biology, at the level of the individuals cell, there is evidence of evolution in that
there are many similarities that can be observed when comparing various cells from different
organisms. Many structures and pathways within the cell are important for life. The more
important and basic to the functioning of the tissues in which cells contribute, the more likely
it will be conserved. For example, the DNA code (the genetic material in the cell) is the very
similar in comparing DNA from different organisms.
In molecular biology, the concept of a molecular clock has been suggested. The molecular
clock related to the average rate in which a gene (or a specific sequence of DNA that encodes
a protein) or protein evolves. Genes evolve at different rates the proteins that they encode.
This is because gene mutations often do not change the protein. But due to mutations, the
genetic sequence of a species changes over time. The more closely related the two species,
the more likely they will have similar sequences of their genetic material, or DNA sequence.
The molecular clock provides relationships between organisms and helps identify the point of
divergence between the two species. Pseudogenes are genes that are part of an organisms
DNA but that have evolved to no longer have important functions. Pseudogenes, therefore,
represent another line of evidence supporting evolution, which is based on concepts derived
from molecular genetics.
Perhaps the most persuasive argument that favors evolution is the fossil record. Paleontology
(the study of fossils) provides a record that many species that are extinct. By techniques such
as carbon dating and studying the placement of fossils within the ground, an age can be
assigned to the fossil. By placing fossils together based on their ages, a gradual change in
form can be identified, which can be carefully compared to species that currently exist.
Although fossil records are incomplete, with many intermediate species missing, careful
analysis of habitat, environmental factors at various timepoints, characteristics of extinct
species, and characteristics of species that currently exist supports theories of evolution
and natural selection.
Biochemistry Evidence
Biochemistry also reveals similarities between organisms of different species. For
example, the metabolism of vastly different organisms is based on the same complex
biochemical compounds. The protein cytochrome c, essential for aerobic respiration,
is one such universal compound. The universality of cytochrome c is evidence that all
aerobic organisms probably descended from a common ancestor that used this
compound for respiration. Certain blood proteins found in almost all organisms give
additional evidence that these organisms descended form a common ancestor. Such
biochemical compounds, including cytochrome c and blood proteins, are so complex
it is unlikely that almost identical compounds would have evolved independently in
widely different organisms. Further studies of cytochrome c in different species
reveal variations in the amino acid sequence of this molecule. For example, the
cytochrome c of monkeys and cows is more similar than the cytochrome c of
monkeys and fish. Such similarities and differences suggest that monkeys and cows
ate more closely related than are monkeys and fish. Scientists have similarly
compared the biochemistry of universal blood proteins. Their studies reveal evidence
of degrees of relatedness between different species. This evidence implies that some
species share a more recent common ancestor than other species do. From such
evidence scientists have inferred the evolutionary relationships between different
species of organisms.
The similarities described above are not the only ones scientists have noticed among
organisms of different species. The image to the left shows that embryos of certain
species develop almost identically, especially in the early stages. Such physical
similarities indicate that there are genetic similarities between the organisms. These
similarities can be considered evidence that the organisms shown probably
descended from a common ancestor.
The similarities between living species-- in ancestry, in homologous and vestigial
structures, in embryological development, and in biochemical compounds-- all could
be explained as extremely remarkable coincidences. However, a far more probable
explanation of these similarities is that species have arisen by descent and
modification from more ancient forms. Additionally, the fossil record contributes
compelling evidence that species have changed over time. The fossil evidence and
evidence from living organisms strongly suggest that species evolve.