Paleoevolution & Fossil Records - 2. Lecture

Paleoevolution & Fossil Records

Fossiles are defined as trace of past life - such as body parts, footprints and moulds, left as indents of organic matter or other kinds of thinkable traces.

Fossilisation can only occur if remains of animals are covered by sediments (sedimentation) before they decay (microbial action, scavengers).
The fossil must be preserved and eventually found. The change for any organism to leave a fossil is therefore generally very unlikely and depends on:
  • where organism lived
    - within sediments better than elsewhere
    - surface sediment > water column
    - marine > terrestrial
  • type of organism
    - large > small
    - have skeletons/shell > soft bodied forms
Geologists use relative and absolute techniques to date events.
Radiometric dating infers the absolute geological time:
It uses radioactive decay of isotopes (rubidium isotope 87Rb decay into strontium isotope 87Sr with a half-life(HL) of 48.6 x 10^9 years)

Radioactive decay proceeds exponentially constant and measurable.

For the identification of organic life, the widely present radioactive isotope carbon 14 is used for direct dating.
Unfortunately the HL is only 5730 years, so only very young fossiles can be identified with it.
Most other isotopes occur only in volcanic rock.
Fossiles can be dated indirectly from those rocks in adjacent layers. Another method for relative dating is the use of standard reference fossiles, which have lived in a well-defined time in the past (problems can occur if they are mistakenly placed in the wrong time).

The Origin of Life
Radiometric dating of lunar and meteorite rocks shows that the age of our solar system and the Earth is about 4.54 billion years.
In the first several 100mio years the Earth was too hot for life.
The oldest dated fossils are about 3.5bio years old, they were found in the Archean Apex Chert in Western Australia (presumed the oldest system of sedimental rocks that could retain fossils at all). They are found in stromatolites http://en.wikipedia.org/wiki/Stromatolite - layered colonial structures formed by photosynthesising cyanobacteria.
These life forms are already cellular and modestly complex life has probably originated several 100 of million years before.
This is remarkable since it leaves only a “short” time window of a few 100mio years from the time when Earth cooled down until first life appeared.

Biological life is defined by two properties:
  1. the ability to an entity of to self-replicate (defining "life")
  2. the presence of a cell (which makes life "biological")
As soon as the first replicator appears, evolution by natural selection starts.
The short time window suggests - that taken into account of present conditions in the young Earth - the origin of life from non-life was “easy”. It remains though, one of the major mysteries of biology.
Usually the “naked” replicator is thought to have evolved first - although recent theories support that the first replicators could have appeared in small iron sulphide cavities, which act as inorganic cells.
Stanley Millers primordial soup theory is no longer favoured - the oceans would have been too dilute for the building blocks ever to meet.
Most scientists today believe that the essential catalytic steps occurred on rock surfaces, possibly using the heat and minerals of hydrothermal vents at the ground of the ocean.

Simple organic molecules can be produced under purely abiotic conditions - the problem of the origin of life is the step from these organic components to the first replicator:
only nucleic acid replicate (in known living systems), but they need proteins to catalyse the reaction.
In return, proper proteins can only be produced if they are encoded by DNA.

A solution could come from RNA - RNA folds to complex 3D structures and has been shown to be able to catalyse a large number of reactions that usually need proteins (including self-splicing and transcription regulation, ribozymes)
However it is not yet established how/if RNA can catalyse its own replication and how the first self-replicating ribzyme came into existence

Procaryotes

First organisms, appeared about 3.8bio years ago.
The sole existence of bacterial life dating from the first 2bio years from the origin of life until first eucaryotes appeared 1.8bio years, is known as the “age of bacteria”.
there are 2 known domains of procaryotes, and an inferred 3. one:
  1. Eubacteria: typical bacteria: E.coli, etc
  2. Archaea: prefer weird environments, methanogens, halophiles, thermacocidophiles: Extremophile bacteria
  3. Urkaryotes: hypothetical ancestors to Eukaryotes
Living eubacteria and archaea are at least as different from each other as either is from eucaryotes. Recent work suggests that some aspects of eucaryotes are closer related to eubacteria and others to archaea, indications for horizontal gene transfer.

Eucaryotes

After the origin of life, it took 2bio years before the first eukaryotic cells evolved (ETA 1.8bio years ago). This could be explained because eukaryotic cells need reasonable high levels of oxygen.
This level first had to be produced by photosyntheses of early procaryotes.
Evidence for the presence of oxygen on Earth is 2.2-2.4bio years ago.

At least 3 major steps had to be taken in the evolution of eucaryotes:
  1. Evolution of the nucleus
  2. Endosymbiosis of mitochondria
  3. Endosymbiosis of chloroplasts
The evolution of eucaryotes is an example how life influences and generates the environmental conditions (high levels of oxygen) for its own further evolution.
Since oxygen is highly reactive, it was probably poisonous for most ancient life form - resulting in the first “mass extinction”.
In retrospect it also provides as a potent energy source that enhanced future evolution, once organisms had evolved respiration.

Numbers of various eucaryotic life forms exploded


Radiations and Extinctions
In a brief period of roughly 50mio years during the Cambrian period suddenly all animal phyla that have fossilizable hard parts, appear:
  • Arthropoda
  • Mollusca
  • Porifera
  • etc.
there are apparently no new body plans after the Cambrian period (Cambrian explosion).
It is suspected that this sudden appearance happened due to a “late arrival”, rapid evolution encouraged by first predators initialising a predator-prey arms race, response to evolution of Hox genes or lift of an environmental constraint (i.e. insufficient atmospheric oxygen).
Or an “early arrival”, which would be a partial taxonomic artefact - many creatures finally got hard parts (shells, chitin, skeletons) and become able to fossilise.

Molecular sequence data indicates that the divergence times of phyla extend far into the Precambrian. In any care there is clear evidence of a huge radiation of morphology in the Cambrian - even if morphological diversity may lag behind genetic diversity.
The fossil record shows frequently radiations of smaller groups (dinosaurs in late Triassic, mammals in the Tertiary).

Just as taxa often suddenly appear, they frequently disappear in the fossil record.
A problem is also the classification - if a later form of a species look very different from the old ones,they may be classified as two different species, although there has never been a speciation event or an extinction (pseudoextinction).
According to Darwin, a species goes extinct when it is out-competed by better adapted forms, however extinctions occur due to different causes, too:
  1. End of Permian Period: massive extinction - especially for marine organisms:
    - 50% of all skeletonized marine families extinct
    - perhaps 96% of all species extinct!
    - Most massive extinction in evolutionary history (cause: climatic ?? due to volcanic
      eruptions/glaciation/continental shift ??)
  2. End of Cretaceous: all dinosaurs (15 families), many marine turtles many marine invertebrates (Ammonoids) go extinct. Some crocodiles slide through (likely cause: meteorite
  3. Further mass extinction: End of Ordovician, late Devonian and end of Triassic
  4. 11,000 years ago, large North American mammals go extinct: sabertooth cats, mammoths, giant bison, giant sloths, etc (likely cause: arrival of humans; probably elevated extinction rates until today)
Mass extinction is clear evident and catastrophic causes (yes, humans) are the likely cause.
An interesting question is wether or to what extent survival is just random in these cases, or wether it’s linked to some evolvable character.
Do taxa extinct because of bad genes or bad luck. Another factor in question is, to what extend also smaller extinction events are due to catastrophic events.

Macroevolution

The evidence of radiations and mass extinctions seems to indicate that evolution, over large time periods is not a smooth and continuous process, but proceeds in bursts of rapid change that are separated by phases of relative stasis.
A further observation that fits this pattern is the lack of transitional forms between species, the diatom http://en.wikipedia.org/wiki/Diatom being an exception.
This does not question Evolution as a branching process, but poses questions about the mode and rates of evolution.
One part being anagenetic change which is expressing the morphological change in a lineage due to the processes of microevolution.
A question being, does macroevolution reduce to microevolution?
  1. Macroevolutionary patterns can be explained by conventional microevolutionary processes: Gaps in fossil records are attributed to its imperfection. The pattern can result from standard microevolutionary processes: stabilising selections towards a constant optimum, produces stasis and if change happens in a relative small geographical range, transitional forms may not be found in fossil record in most other regions - even if it’s complete. 
  2. The “traditional neo-Darwinain” view has been questioned by the claim that macroevolution is (at least partly) governed by other processes than microevolution. Prominently the idea of Punctuated Equilibrium, states the claim that the pattern of stasis interrupted by bursts of rapid morphological change (=punctuation events) is real and no artifact of an imperfect fossil record: “gaps aren’t gaps in the data, they are the data”.
Eldredge & Gould (Punctuated Equilibrium) claim that stasis reflects constraints.
In part, the genome is tied up in “coadapted gene complexes”. If a set of genes all interact and fit together, it is not possible to change just a single gene without compromising the effect of the other genes. The genes effectively constrain each other.
During punctuation events, these constraints are broken, allowing for rapid evolution.
E&G’s mechanism for punctuation builds upon Ernst Mayr’s “genetic revolution” theory of speciation.

Speciation happens rapidly in small isolated populations where genetic drift breaks down coadapted gene complexes.
New ones are formed; the newly arisen phenotypes represent new biological species.
Since this happens rapidly and in a small area few or no fossil remains record the transformation.
According to this morphological change is coupled to speciation.
Individual species are not static, the immediate consequence is that trends in morphology are not the result of anagenetic change within lineages, but of selection among species i.e. species selection:
If big species speciate faster than small species, or go less often extinct - we will see a long-term increase in body size. This pattern is not due to change within lineages.

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