ORIGIN OF LIFE AND THE FOSSIL RECORD
A brief review of salient points regarding the origin of life:
Cosmic calendar: Earth formed 4.6 billion years ago; there has been
a long time for life to evolve. It took about a billion years to get through
the early stages of chemical evolution such that there is some form of
self-replicating system (e.g., a primitive living thing in its simplest
definition). Miller experiments lead to formation of amino acids
under lab conditions simulating a primitive earth atmosphere. Subsequent
reactions could produce short polymers of the amino acids. When polymers
are heated to 130°C to 180°C and then cooled in water to 25°C
- 0°C proteinoid microspheres form. These provide evidence that
simple cells could have formed from some of the earliest compounds.
Progress has also been made on the synthesis of nucleic acids. One significant
bit of evidence, much further down the line, was the discovery of catalytic
RNAs that performed enzyme like functions. This, and other evidence,
suggested that RNA may be ancestral and DNA is a derived molecule for the
storage of genetic material.
By 3.2 billion years ago, first procaryotes (Bacteria, blue green algae).
By 2.5 - 2.0 billion years ago, communities of procaryotes emerge. e.g.
Stromatolites as colonies of Blue green algae, formed biosedimentary
domes of calcium carbonate = some of the earliest fossils. Photosynthetic
bacteria have significant effect on the earth's atmosphere and the subsequent
evolution of life. Blue green algae are photosynthetic and produce oxygen
as a waste product. This was initially a poisonous molecule (as environment
was an anoxic one) Lead to the production of an oxidizing atmosphere.
Large amounts of Oxygen oxidize the vast quantities of dissolved iron
in the oceans: i.e., the oceans "rust." This counteracts the
poisonous atmosphere problem, but only until the reservoir of iron is depleted
and the iron settles out as the banded ironstone formation = layers
of iron which form iron ore deposits. Ultimately, with the absence of iron
to oxidize, the oxygen builds in the atmosphere and produces an ozone layer.
This is a singular event which eukaryotes will ultimately take advantage
of in the form of oxidative respiration. Subsequent cellular (at this time
= organismal) evolution is contingent on this singular event.
If we started earth over again, would this event re-occur? at the same
time?, if not would we have evolved???
1.5 Billion years ago, a diverse flora of Eukaryotes present
as asexual species. 1.4 By eukaryotic algae present. First metazoans
seen in the Ediacara fauna for Australia (680 MyrBP).
Before considering the diversity of fossils we need to think about how representative the fossils are of past life which is largely a function of what gets preserved and where it might get preserved.
What gets preserved? Hard parts, and other parts that can be
mineralized. Sequence of events from death to scavenging to decay to covering
with soil. Example from heard of elephants: "wet" stage
= two weeks (too much tissue for vultures so many invertebrates
helped out). By the end of the third week, Dermestid beetles had
removed all the skin and sinew from the bones. Within five weeks
the temperature fluctuations caused the bones to crack and flake. Within
one year the skeletons were completely disarticulated. Within two
years many bones were covered with soil. Current day events can shed light
on the fossilization process.
Fossilization: percolation of mineral grains (e.g. calcium carbonate)
into interstitial spaces of hard part tissue. In bone the mineral is calcium
phosphate which can incorporate fluorine, present in minute amounts in
water, into the Calcium Phosphate to produce a crystal more resistant to
Death assemblage: become fossils at a site away from their actual
habitat due to death and transport to an area. Life assemblage:
organisms preserved in their natural habitat. Obvious example: If large
mammal bones were found scattered among fossil fish, one presumably would
not invoke the existence of primitive mammals that walked on lake
or ocean floors!
Environments: fossils are generally restricted to areas of deposition.
Upland areas less likely to preserve fossils: more erosion.
In deserts material is covered by sand and has a good chance
of being fossilized. In shallow seas sediment is being deposited
and can cover skeletons. Some of the best fossil assemblages are from shallow
sea deposits, lake beds, outwash plains from periodic river floodings,
Ediacara fauna (640 MyBP) Many forms that bear some resemblance
to modern phyla. Appears as if it were a major "evolutionary experiment"
that did not work as it appears that none of their representatives made
it into the Cambrian.
Burgess shale (530 MyBP, British Columbian rockies) Discovered
in 1909 by Charles Doolittle Walcott: remarkable diversity of many different
forms. Some of these are represented today many others are not (about 15-20
distinct, and now extinct, phyla). e.g. Hallucigenia, Opabinia,
Yohoia, Pikaia (first chordate), etc. Nicely illustrate the nature
of Contingency (see S. J. Gould, Wonderful Life, 1989, Norton).
The "iconography of the cone" led Walcott to erroneous
pigeonholing of the Burgess shale organisms into "known" groups.
The more appropriate image is "decimation" where only
some organisms get through alive and those that do may be simply lucky.
Harry Whittington in the 1960s and 1970s with Simon Conway Morris in the
mid to late 1970s reanalyzed Walcott's collections. Concluded that there
were many unique morphologies so new that they deserve the status of new
phyla!. Many of Walcott's classifications were wrong. What would have
happened if Pikaia had not made it through the "decimation"?
(would you be here reading this? Another example of contingency).
Other important points in interpreting the fossil record: Dating fossils
requires radiometric dating of associated igneous rock. (sedimentary
rock is of highly mixed origin). Moreover, fossils and the bed in which
they lay have been reworked and redeposited. Careful stratigraphy and analyses
of surrounding strata must be done to provide meaningful data about the
relative and absolute ages of fossils. Gaps in the record.
The nature of the fossilization process almost assures that there will
be gaps in the fossil record. We have to live with it.
What do we know about fossil organisms? Certain associated information
allows informed speculation about the biology of fossil organisms. Large
dinosaurs that left tracks without tail dragging marks suggest an active
lifestyle? (other fossil remains do show clear evidence of tail dragging
and footprints). Other assemblages show fossil bones of adults associated
with nest sites and eggs: suggests parental care? Simple footprints
may seem like a cute form of fossil evidence. Actually a lot can
be learned about the organisms: one can corroborate estimates of the animal's
size; one can measure distance between prints and obtain information about
gait, travel speeds, etc.; these interpretations further dictate a host
of different physiological processes that might be able to sustain such
a manner of locomotion. These types of issues are the main point
of this lecture: from a small amount of fossil information, certain biological
interpretations are implied simply by the necessary biological attributes
that go along with a given footprint size, shape, etc.
Fossils can help define ancestral character states and thus help clarify relationships of extant organisms. However, this cannot be done without the extant organism's character states (i.e. fossils alone aren't much help. Is Archaeopteryx birdlike enough to be considered a bird ancestor?