one knows when humans first discovered fossils, but we can be
fairly certain that proper interpretation in a non-superstitious,
scientific context did not begin until a few hundred years ago,
give or take several decades. The word fossil conjures in most
people an immediate image of a rock. Indeed, fossils are usually
rocks of sedimentary origin containing mineralized traces of past
organic life on earth. Viewed more broadly, fossils are a recorded
history of life on earth, or as Darwin put it, “a history
of the world imperfectly kept … only here and there a short
chapter has been preserved; and of each page only here and there
a few lines”.
naturalists well understood the similarities and differences of
living species leading Linnaeus to develop a hierarchical classification
system still in use today. (17) It was Darwin and his contemporaries
who first linked the hierarchical structure of the great tree
of life in living organisms with the then very sparse fossil record.
Darwin eloquently described a process of descent with modification,
or evolution, whereby organisms either adapt to natural and changing
environmental pressures, or they perish.
I am initially
constraining editorial boldness to the discussion page, because
I think the two paragraphs that follow (offered without even my
own copy editing) markedly expand the scope of the fossils page,
which is my suggestion, but I worry about drifting too far a field.
So, I look for feedback (concurrence and tomatoes) from what I
think are several others dedicated to improving the page. I love
the title “Developments in interpretation of the fossil
record” that enables chronological storytelling. For references,
I drew upon Knoll, Conway-Morris, Gould, Fortey and a couple of
websites, all of which are enjoyable, non-recondite reading. I
have also purposefully included keywords (e.g., tectonics; extinction
events; Cambrian explosion) to facilitate page development..
paleontology is too often thought of as the study of dinosaur
bones because that is what captures the attention of the public
and the news-reporting venues. The modern study of fossils actually
has interdisciplinary linkages to many other scientific fields
such as molecular biology, geochemistry, and evolutionary biology.
The dinosaurs existed on an earth not greatly different than our
own, and arguably their lineage is of relatively minor scientific
importance. The preceding Paleozoic period was markedly different.
It began with no terrestrial life, a highly oxygenated atmosphere,
and with the progenitors for what would evolve to be the life
forms we recognize today, as well as others that did not persist.
Paleozoic fossils are testimony to an era of dramatic change to
both earth and its life forms. The drama of the Paleozoic, beginning
with the Cambrian Explosion exemplifies the coevolution of earth
and life that has continued unabated since life appeared. “The
evolutionary epic recorded by fossils reflects, as much as anything
else, the continuing interplay between genetic possibility and
ecological opportunity” (pg 5 of Knoll). The earth’s
climate, tectonics, atmosphere, oceans, and periodic disasters
invoke the primary selective pressures on all organisms to which
they must adapt or perish. Many extinction events punctuate geological
history, leaving environmental voids in which lineages appear
and divide. The life forms, in turn, greatly affect earth’s
environments. The fossil record encodes the inextricably linked
coevolution of life and planet, albeit with diminishing clarity
prior to the Cambrian where mostly microscopic life left meager
paleontology and evolutionary biology share a goal of unfolding
the tree of life, which inevitably leads backwards in time to
the microscopic life of the Precambrian when cell structure and
functions evolved. Earth’s deep time in the Proterozoic
and deeper still in the Archaean is only “recounted by microscopic
fossils and subtle chemical signals”. During deep time,
life only existed where it first appeared, in the sea, and most
extant life yet exists in the sea. Even now “the land-based
animals each carry with them a miniature ocean, pulsing in their
cells and circulatory systems. All life, including human, could
be viewed as containers of sea water with the same mineral constituency
as the oceans and a dynamic dispersion of molecules that perform
the biological processes that constitute life”. Molecular
biologists, using phylogenetics, can compare DNA nucleotide and
protein amino acid sequence similarity to infer taxonomy and evolutionary
distances among organisms, but with limited statistical confidence.
The study of fossils, on the other hand, can more specifically
pin point when and in what organism branching occurred in the
tree of life. Modern phylogenetics and paleontology act together
to clarify sciences dim view of the appearance life and its evolution.
Charles Darwin was the first to recognize that the systematic
hierarchy represented a rough approximation of evolutionary history.
However, it was not until the 1950s that the German entomologist
Willi Hennig proposed that systematics should reflect the known
evolutionary history of lineages as closely as possible, an approach
he called phylogenetic systematics. The followers of Hennig were
disparagingly referred to as "cladists" by his opponents,
because of the emphasis on recognizing only monophyletic groups,
a group plus all of its descendents, or clades. However, the cladists
quickly adopted that term as a helpful label, and nowadays, cladistic
approaches to systematics are used routinely.
* Monophyletic: two or more DNA sequences that are derived from
a single common ancestral DNA sequence.
* Clade: a group of monophyletic DNA sequences that make up
all of the sequences included in the analysis that are descended
from a particular common ancestral sequence.
* Parsimony: an approach that decides between different tree
topologies by identifying the one that involves the shortest evolutionary
pathway. This is the pathway that requires the smallest number
of nucleotide changes to go from the ancestral sequence, at the
root of the tree, to all of the present-day sequences that have
* Molecular Clock Hypothesis: states that nucleotide substitutions,
or amino acid substitutions if proteins are being compared, occur
at a constant rate, that is, the degree of difference between
two sequences can be used to assign a date to the time at which
their ancestral sequence diverged. The rate of molecular change
differs among groups of organisms, among genes, and even among
different parts of the same gene. Furthermore, molecular
clocks require calibration with fossils to determine timing of
origin of clades, and thus their accuracy is crucially dependent
on the fossil record, or lack thereof, for the groups under study.
Fossil DNA older than about 25,000–50,000 years is virtually
empty of phylogenetic signal except in rare instances, and therefore
traditional morphological studies of extinct and extant organisms
remain a crucial component of phylogenetic analysis.
The use of fossils in the phylogenetics of extant clades traditionally
has been a contentious issue. Fossils usually are relatively incomplete,
and their use commonly leads to an increase in the number of equally
most parsimonious trees and a decrease in the resolution of phylogenies.
Fossils alone, however, provide certain kinds of information about
the biological history of a clade, and computer simulations have
shown that even highly incomplete material can, under certain
circumstances, increase the accuracy of a phylogeny, rather than
Examination of the fossil record allows us to: 1) make inferences
about the nature of past environments, 2) reconstruct ancient
biological communities, 3) estimate abundance of component species,
4) examine rates of evolutionary change and 5) understand the
origin of the assemblage of extant organisms. However, the interpreting
the fossil record is not trivial. Some species, due to structure
and habitat, have outstanding fossil records, other species due
to their structure or habitat may have a poor record. While a
fossil can be very helpful in phylogenetic analysis, fossils are
not required for phylogenetic reconstruction.
Wiley, E.O., D. Siegel-Causey, D.R. Brooks, and V.A. Funk. 1991.
The Compleat Cladist: A Primer of Phylogenetic Procedures. The
University of Kansas Museum of Natural History, special publication
no. 19, Lawrence.
data have a role in phylogenetic analysis