Lecture 23: Origin of Life, Fossil record, Diversity
Contents
Lecture 23: Origin of Life, Fossil record, Diversity¶
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 was the discovery of catalytic RNAs that performed enzyme like functions. This, and other lines of 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 prokaryotes (Bacteria, blue green algae) are seen in the fossil record. By 2.5 - 2.0 billion years ago, communities of prokaryotes emerge e.g., Stromatolites as colonies of blue green algae, formed bio-sedimentary domes of calcium carbonate. these are among 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]{.ul}. 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 Ediacaran fauna from Australia (680 MyrBP) and Mexico.
Before considering the diversity of fossil life we should consider how good the fossil record is. The fossil record is largely biased what gets preserved and what environments allow for preservation (study of Taphonomy).
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 erosion.
Death assemblage: often fossils occur 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, etc.
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. There are also simple 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.
History of Life on Earth– the Phanerozoic¶
Precambrian¶
Ediacaran fauna(640 MyBP; Pre-Cambrian) 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.
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 in evolution (see S. J. Gould, [Wonderful Life]{.ul}, 1989, Norton). The “iconography of the cone” as Gould put it led Walcott to erroneous pigeonholing of the Burgess shale organisms into “known” groups. The common view of evolution Gould points out was one of increasing biodiversity over time, i.e. a cone increasing from past to present. A more appropriate image is “decimation” where only some organisms get through alive and those that do may be simply lucky and those few stocks that survive then diversify. 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).
Ordovician¶
485-443 Million years ago. Named after rocks in Northern Wales. Associated with further diversification within the oceans. great Ordovician biodiversification event (GOBE). One of the most potent cladogenic events in the history of life. Filled in global diversity on the order of 7x greater than in the Cambrian. Fewer new phyla added during this period but more of lesser taxonomic rank (e.g., orders, families).
First land plants. In the oceans cephalopods, crinoids (stalked filter feeders), and brachiopods (primative clam like things)!
Silurian¶
443-419 Million years ago. First vascular land plants. Eurypterans (giant sea scorpions). First jawed fishes.
Devonian¶
419-358 Million years ago. Age of Fishes. Placoderms still commons but now see major radiation in fishes. Sharks originate. Also lobefinned fishes (sarcopterygeans) give rise to fist tetrapod animals that crawled on to land. These first tetrapods are generally quite large amphibians, that are quite fish-like. e.g., Icthyostega, Tiktaalik. First real terrestrial ecosystems at this point.
Carboniferous¶
358-298 Million years ago. During Devonian and early Carboniferous major forests form. O2 levels quite high at this time. Leads to GIANT arthropod insects– as they are limited by diffusion of O2. On land we see first major split of Denovian amphibian fauna into stem amniotes and very late in the Carboniferous, early possible split of synapsids and diapsids (the lineages that lead to mammals and lizards and snakes respectively). Amniote egg, with its water proof membranes, is key innovation to allow invasion of terrestrial environments.
Permian¶
298-251 Million years ago. last epoch in Paleozoic. Rise of the reptiles. Major radiation of amniotes at this point. Synapsid, diapsid, and anapsid (turtles) lineages well defined. Permian synapsids early on are lizard-like – think sailed reptiles like Dimetrodon and other pelycosaurs. Therapsids, more derived lineage from pelycosaurs, seem more mammal like (may have had butts! a uniquely mammalian feature). On diapsid side we see reintroduction to the sea with aquatic reptile lineages like mesosaurs. Permian ends with largest mass extinction known – Permian Triassic event. PT event kills 96% of marine taxa!!
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?
Mesozoic¶
The Mesozoic era begins with the Triassic period, and signals what is a more familiar flora and fauna to most students. This includes the rise of Dinosaurs and the origin of mammal ancestors. For our purposes we will spend a bit of time on dinos.
Dinosaurs¶
What is a Dinosaur? 1) occured from the mid-late Triassic to the end of the Cretaceous (220 mil. years ago, mya to 65 mya) 2) they are “reptiles”, (but as we know this is not a monophyletic group), 3) they were terrestrial (excludes marine plesiosaurs, ichthyosaurs, mososaurs, 4) Upright pillared legs (an obvious structural “necessity” given their weight. Only the latter is a good “defining” character, cladistically, since there are plenty of other organisms that fit descriptions 1-3 that are not dinosaurs. Moreover, some mammals and birds have pillared legs.
There are two general groups of dinosaurs based on hip morphology The Saurischia (reptile-hipped) and the Ornithischia (bird-hipped). In both groups the ilium and the ischium have relatively similar forms, but in the Ornithiscia, the pubis has a narrow rod-shaped extension running ventrally and posteriorly along the ventral side of the ischium. In the Saurischia, the pubis extends ventrally and anteriorly and only articulates with the ischium (and ilium) to form the hip “socket”. Most of the Ornithischia also have a horn covered beak and bony rods and vertebral spines.
Within each of these two major groups there are further distinct types. Within the Saurischia there are two major groups the Therapods (“beast-foot”) and the Sauropods. Typical Therapods are Tyrannosaurus rex, Deinonychus. These are carnivorous, have bird-like feet, bodies are balanced at the hip with a long powerful tail. Within the Sauropods are the huge species such as Apatosaurus, Brontosaurus, and Brachiosaurus. These walked on all fours, had long whip-like tails and were herbivorous.
Within the Ornithischia there are five major groups:
Ornithopods e.g. Hadrosaurs, duck-billed dinosaurs
Ceratopians e.g., horned and frilled dinos such as Triceratops
Pachycephalosaurs with large bone-filled heads
Stegosaurs (e.g., Stegosaurus) with large dorsal spines of disputed use in thermoregulation
Ankylosaurs heavily armored and abundant in late Cretaceous
The only living relatives of Dinosaurs are Birds. From the names of the two groups one might expect that the birds descended from the Ornithischians. This is not the case. Birds are related to Therapod dinosaurs. The living sister taxon to birds are Crocodylians; how do they fit in? You probably think of Pterosaurs as dinosaurs, too, but they are not. Below is a simplified cladogram of relationships.