Evidence for evolution by way of Common Descent
Fossils: The fossil record tells us the history of the succession of species that have lived, but which are
now extinct. The fossil record is incomplete. That is to say that we do not have fossils of many of the
transitional forms that would have been necessary intermediates between species. Soft-bodied animals
don't fossilize well at all. In fact, even for organisms with hard parts (even rigid cell walls), the
conditions must be absolutely perfect (sudden death, no oxygen, proper pH, proper sediments, etc.). Still,
the fossils that we do have tell us a rather vivid story of the history of life on earth, and we can see that the
succession of life has led to the extant species we see today. The fossil record also tells us of mass
extinction events (5 or 6) that have occurred since the Cambrian Explosion about 555 million years ago.
Radiometric dating of rocks tells us within a margin of error of less than 2-3 % the age of the fossils.
Comparative Anatomy: By comparing the anatomical features found in fossilized, extinct animals with
each other and with living species, a degree of relatedness can be estimated. The lineage of vertebrate
animals is very clearly drawn with comparative anatomy. While we may look quite different from pigs,
comparative anatomy reveals striking similarities. So comparative anatomy is used between extinct
species, extinct and extant species, and between different extant species to determine the degree of
relatedness and estimate the branch points on the Tree of Life that represent the common ancestor of the
species being studied.
Comparative Embryology: In some lineages of multicellular species, it appears that changes
(mutations) during embryological development have profound effects on the adult form. Consider that an
embryonic cell may be one of thousands, and differentiation of a mutated cell can be manifested in
billions of cells in the adult. By comparing the developmental sequence of different animal embryos,
scientists can make inferences about the evolutionary relatedness. The results very closely mirror the
patterns evident in the fossil record and comparative anatomy of adult forms.
Molecular Evidence: The three lines of evidence described above have provided us with a very
compelling picture of the lineage of multicellular organisms, but even putting all three together does not
match the evidence that is present in the DNA of living organisms. The degree to which the sequence of
A,T,C, and G in the DNA of living species is homologous (the degree of similarity) is a spot on indicator
of how closely related species are. Indeed, sequence homology can answer the question of how closely
any two humans are. From sequence homology data, we can closely estimate how many generations any
two humans must go back in their genealogy to find a common ancestor. Taken to the next step and the
step after that, sequence homology can be used to construct a Tree of Life that is more accurate in its
detail than that of the ToL constructed without molecular evidence, but the basic framework of the tree
(with the multicellulars anyway) is remarkably close to the one created without molecular evidence.
Molecular studies have also allowed scientists for the first time to examine explore the lineages of
microbes. Data is coming faster than it can be analyzed.
ERVs and Pseudogenes: Endogenous retro-viruses: (HIV is a retrovirus; its genome is RNA,
and it must create a complementary DNA copy of its genome to incorporate itself into an organisms
DNA. To do so requires a gene - reverse transcriptase - that is unique to retroviruses.) Because of the
reverse transcriptase gene, ERVs are rather easy to find. Retroviruses have infected our ancestors and
ancestor species for millions of years. We have over 30,000 ERVs in our genome, and at least 12 ERVs
in common with chimps - our closest relatives. These are located in the same place in the same
chromosome. We have fewer in common with the other great apes (gorillas and orangutans).
Pseudogenes are non-functional counterparts to genes that still function. Their function has been lost as a
result of mutation. We have now found pseudogenes in other mammals and once again, the more of these
pseudogenes common to different species, the more recent is their common ancestor.
Homeobox sequences and HOX Genes: Scientists working with the rapidly reproducing fruit fly
began playing around with embryos, moving cells around and mutating genes. What they discovered was
a very small set of adjacent genes (Hox genes) that code for the body segments. They were astounded
when the EXACT same genes were found in mice controlling the development of the mouse embryo.
Furthermore, these genes could be move from fly to mouse or vice versa, and in both cases they
performed just as well as the original fly or mouse genes! We now know that all animals with body
segments (all of the vertebrates and most of the invertebrates) have these same Hox genes. Common
ancestry.
Piecemeal or Whole Genome: Working within a specific lineage, scientists can sequence specific
regions of the genome and use computers to determine the degree of sequence homology. In vertebrates,
the genes coding for hemoglobin are very useful. But our common ancestor with plants goes much
further back in time and plants don't have hemoglobin. As difficult as it may seem, we do have many,
many genes in common with plants, fungi, amoeba, and even bacteria. The genes common to ALL life
are associated with the molecular functioning that is common to all life. The universal genetic code
strongly infers the existence (in the distant past) of a Last Universal Common Ancestor - LUCA. The
genes coding for the translation of the universal genetic code have been studied for the past 30 years, and
this has allowed us to construct a Tree of Life that includes our microbial relatives for the first time.
Previously, lineages of microorganisms were basically impossible to work out. So in many cases DNA
sequences of tiny fractions of the genome are all that is necessary. For example, forensic scientists can
determine the identity of a suspect from hair, semen, saliva, etc. O.J. Simpson's DNA was found in blood
at the scene of the crime (he murdered his wife and her friend), but the jury was ignorant of the
significance of the DNA evidence and OJ had enough money to hire the "best" lawyers… so he was
acquitted. OK, that has nothing to do with evolution, but you get the idea…. DNA evidence is extremely
significant and undeniable. If a boy's paternity is in question, sequencing a bit of his Y chromosome as
well as that of the suspect "dad" will put an end to the debate.
Whole genome sequences are much more laborious than sequencing small regions of the genome.
Twenty years ago we completed a whole genome sequence of a bacterium. WOW! Now we have whole
genome sequences of about 300 species (including humans), and hundreds more in the works even as
sequencing technology and computational technology are advancing rapidly.
Theories can be used to make predictions. The theory of evolution by common descent predicts the
existence of common DNA sequences - that is once we learned about DNA and how to sequence it. So
scientists set about looking for homologous sequences and found even more than they expected.
Evidence for evolution by common ancestry is now irrefutable.