Lecture 10: February 26

• Fortey, R., 2000.  Exploding trilobites, Chapter 5 in 'Trilobite: eyewitness to evolution'. p. 120-145
• P. F. Hoffman and D. P. Schrag, 2000, Snowball Earth, Scientific American, January 2000 (web version)
• D. H. Erwin, 2001. 1.2. The Cambrian Radiation. In: D. E. G. Briggs and P. R. Crowther, eds., Paleobiology II, Blackwell Science.

There are many web sites with information on the Burgess Shale; links are given in this web-enhanced version of a paper by S. J. Gould, 'Play it again. life', published in Natural History in February 1986.

What is the Cambrian Explosion?

• A geologically short period (maximum 25 million years) during which bilaterally symmetrical animal Phyla with skeletons first show up in the fossil record, and rapidly diversify
• Since then no new Phyla originated, none became extinct (as far as we know..)

BUT there WERE large, multicellular 'animals' around BEFORE the Cambrian Explosion, called Vendobionta or Ediacara faunas.

What was the (hypothetical) ancestor of all biserially symmetrical animals? (Urbilateria)

• Share characters of protostomate and deuterostomate animals
• Have all genes shared by Arthropoda (fruit flies as examples of Protostomata) and Chordates (mice as examples of Deuterostomata)
• photoreception organ (e.g., genes called Pax-6 trigger formation of eyes in both groups)
• appendages (maybe tentacular feeding structure?; Hox group genes trigger formation of legs in both groups)
• heart (contractile muscle)
• some type of segmentation

• Fossils of multicellular animals found in sediments of late Precambrian age; have no mouth
• Some were jelly fish, sponges (animals without without organ-level organization)
• Others are like 'quilted air mattressess'
• Take up food by diffusion from water surrounding them; large, flat shapes
• Have many bacterial symbionts; may have been similar to present day organisms such as the green slime balls, (Ophridium versatile), complex colonies of uniserial organisms.
• Seafloor covered with sticky slimy bacterial mats, no burrowing (no animals with muscles)

"The Garden of Ediacara"

• Ediacara ecosystems consisted of:
  1. Jellyfish or sea anemones (related to modern Cnidaria), such as Nemiania (this fossil has been interpreted as a jellyfish, but nowadays many people think it is the basal impression, like a 'footprint', of a sea-anemone like organisms, or of a frond-like organism as described below), Eoporpita, Cyclomedusa, or possible Echinodermata such as Tribrachidium, or possible relatives of Mollusca such as Kimberella.
  2. Frond-like organisms, possibly related to modern soft corals of the Phylum Cnidaria and worms of the Phylum Annelida such as Dickinsonia, or possibly even Arthropoda such as Spriggina.
  3. Trace fossils: burrows in mud, maybe made by Annelida. Must have been bilaterally symmetric forms, could dig, had guts.
  4. Unusual forms (quilted mattress structure, no mouth, gut or anus), such as Charnia or Pteridinium. Note: there is no agreement on whether such forms as Spriggina and Charnia belong in group 2 or group 4.
• All life forms were either suspension feeders (taking particles from water), detritus feeders (picking up bacteria from sea floor), or lived on bacterial symbionts
• No predators (no jaws), no burrowing up old organic matter
• Cropping principle: floras/faunas which are 'cropped' (i.e., eaten by other organisms) tend to be more diverse than non-cropped associations. There are additional niches in cropped ecosystems: organisms adapt in order to escape being eaten (e.g., bad taste, spines). Idea is that when animals (e.g., bilaterally symmetrical animals) evolved and some became predators, the diversity of the overall world's evosystems increased.

Discrepancy between fossil and molecular data.

• Fossils: separation between the two groups predated the Cambrian explosion; occurred at about 670 Ma. (if such forms as Kimberella are molluscs and Tribrachidium is an echinoderms, protostomates and deuterostomates existed at the time that these forms were present in the Ediacara faunas of that age).
• Molecules: separation occurred at the very latest 700 Ma ago, but more probably as long ago as 1200-1500 Ma.

Why such a very large discrepancy?

• Assumptions regarding rates of mutation may be wrong, especially since we may be looking at very early development of regulatory genes as we know them
• Fossil record highly incomplete (but we have fossils of Ediacarans, and these faunas do not have forms that are inequivocally 'modern Phyla'

Possible Solutions:

• Assumption of constant rate of mutation may be wrong; evidence available that it is wrong for some genes.
• Early animals had no shells, leave no fossils. BUT there are also no traces of worms burrowing (for burroweing one needs a body cavity, i.e., be a bilaterian animal). And Vendobionta had no hard parts but did fossilize.
• Early animals were very small (interstices between grains of sand) - but very small fossils are known and have been looked for.
• Maybe we did not look at right spot (we looked in shallow oceans)

• Proto-deuterostomates split long BEFORE the Cambrian explosion (543 Ma)
• Vendobionta: 670 Ma on, maybe older. Why do we not find the ancestors of proto-deuterostomates in these same deposits? Vendobionta were soft bodied, difficult to fossilize.
• Deuterostomates and both Protostomate groups rapidly diversified in the fossil record (i.e., their hard parts) early Cambrian, as did unicellular algae (Acritarchs)
• Several unrelated groups all show rapid evolution at the same time:  process triggered by some environmental process? By interaction between biota?

Vendobionta survived evolution of 'modern Phyla'; why did they become extinct?

• Eaten by organisms which developed jaws (internal)
• Some environmental disaster (external)

Life on Earth

• Over 3.85 billion years, life on Earth never became extinct
• All that time there must have been liquid water on Earth
• Surface temperature of Earth between 0 and 100^oC.

The Earth's thermostat: recycling of material by subduction deep in Earth; But: did this thermostat work when life was not very abundant?? Maybe not!

• Earth's climate fluctuated wildly, 750-580 million years ago; carbon isotope data suggest that living organisms were involved, at some times depositing much organic matter in rocks, at others not.
• Cold phases: at least 2 'Snowball Earth' phases; the Marinoan and the Sturtian; 720-695 Ma (Sturtian), around 610-590 Ma (Marinoan); possibly minor third one at about 570Ma
• Rocks that indicate presence of glaciers, at sea level, at low latitudes: glaciers, ice berg

1. Snowball Earth, step 1:
   • Breakup of a single land mass 770 Ma: small continents near equator, high rainfall.
   • Rapid weathering of rocks (wet); CO₂ out of atmosphere; organic productivity: organic matter stores in rocks.
   • Ice packs at poles; ice reflects solar energy-> even colder  ( + feedback)
   • Planet iced over within a millennium.
2. Snowbal Earth, step 2:
   • Global temperatures to maybe 50^oC ; oceans ice over (to what depth?), limited by heat from Earth's interior.
   • Much life dies; survives at hot springs (but ends deposition of much organic matter in rocks).
   • Cold and dry: stops growth of land glaciers, deserts; dry: not much weathering.
   • CO₂ emitted from volcanoes is not removed from the atmosphere; planet warms, sea ice thins.
3. Snowball Earth, step 3:
   • CO₂ increases 1000 fold in ~10 million years (volcanoes): warming.
   • Moisture feeds land glaciers; they grow
   • Open water in tropics: less heat reflected back into space -> fast rise in global temperatures.
   • In centuries: a brutally hot, wet world
4. Snowball Earth, step 4:
   • Oceans thaw, sea water evaporates, with CO₂ produces intense greenhouse
   • Temperatures to > 50^oC, intense evaporation, rainfall.
   • Rivers: ions into the oceans, carbonate sediments.
   • New life-forms (prolonged genetic isolation, selective pressure) populate the world.

• Good evidence for extreme climate swings (hot to cold), cold extreme (ice in tropics)
• The timing of such episodes is not clear; neither is it clear how many there were (estimates range from 2 to more than 4)
• Not clear to which depths, over how much area, oceans were frozen
• Theory very vague as to life forms, and timing of life forms not clear either because of discrepancy between fossil and molecular records. At 750-580 million years ago there were Eukaryotes, Prokaryotes, multicellular organisms (Vendobionta: 670-545 Ma).
• Split Protostomes-Deuterostomes almost certainly was before Snowball Earth Episodes (around 700 Ma); 'Cambrian explosion' almost certainly considerably after (543 Ma; latest age quoted for last Snowball episodes ~570 Ma): too long a lag time
• If oceans frozen until depths of 1000 m: Vendobionta would not have survived until after Cambrian revolution- which they did.

• Tropic Oceans at least partially ice-free
• Plankton heaven: lots of nutrients => lots of phytoplankton => lots of zooplankton
• Many larvae from present day marine animals planktonic: ancestors planktonic? Maybe what happened at the Cambrian Explosion was the evolution of the presently known adult stage; possibly linked to evolution of Hox genes which regulate growth patterns.
• Open ocean rocks not common; would explain why fossils from before Cambrian revolution (soft-bodied larvae) never found
• Strong competition in few areas of ice-free surface-layers; selection pressure to dwell on ocean floor

• Internal factors:
  • evolution of genes to make additional life stage possible, get organism to develop correctly
• External factors:
  • extreme ice ages, concentrating life in tropics
  • competition in small regions of world where plankton could live (no ice)

What is the Cambrian Explosion?

"The Cambrian explosion is the historical product of the interplay between genetic possibility and environmental opportunity, amplified by ecological interactions to extend across all of biology". A. H. Knoll and S. B. Carroll, 1999, Science 284, p. 2129-2137.

"The Cambrian explosion is the historical product of the interplay between genetic possibility (=development of regulatory genes) and environmental opportunity (=possibility to move to sea floor from plankton) , amplified by ecological interactions (=strong competitions on 'Slushball Earth'; development of predators) to extend across all of biology".