1. Chapter 3 What the rocks say: how geology and paleontology reveal the history of life

2. Assigned reading
Assigned reading: Chapter 3 of your text book

3. Darwin recognized that evolution required an old earth
Darwin argued that earth could be billions of years old based on geological observations he made.
Disputed by Lord Kelvin
Proposed earth was no more than 20 million years old based on the rates of cooling of rocks
Kelvin’s calculations later proven to be flawed because they ignored heat generated by radioactive decay.

4. Radioactive decay
Chemical elements are defined by the number of protons they possess in their nucleus. They also possess neutrons in their nucleus, but the number of neutrons may differ between different versions of an element, which are referred to as isotopes.
For example, three isotopes of carbon:
Carbon-12 has 6 protons and 6 neutrons.
Carbon-13 has 6 protons and 7 neutrons.
Carbon-14 has 6 protons and 8 neutrons.

5. Radioactive decay
Some elements are unstable and spontaneously shed protons and neutrons (releasing energy as heat in the process). These are called radioactive elements.
E.g. Uranium-238 breaks down through a series of [also unstable] intermediate elements to Lead-206.

6. Half-lifes
Each radioactive isotope has a characteristic rate of decay called it’s half-life.
Half-life is the time taken for half of the mass of an element to decay.
For example, Uranium-238 has a half life of 4.47 billion years.

7. Radioactive clocks
By examining the ratios of a radioactive element and its decay product in a piece of rock it is possible for scientists to estimate that rock’s age.
Thanks to radioactive clocks we can estimate the age of the earth as about 4.6 billion years.

8. Radiometric dating indicates that the earth is 4.6 billion years old
See text chapter 3 page 62 for detailed explanation of this slide

9. The fossil record is incomplete
Fossils provide insight into early life forms, but the fossil record preserves evidence of only a tiny fraction of living things.
Most organisms do not fossilize because they are consumed (by scavengers and microorganisms) before they can be fossilized.

10. Fossilization
Fossilization generally requires rapid covering of the organism by sediment (e.g. in a lake) where over time the bones are replaced by minerals as water percolates through them.
In cases where no oxygen is present, microbes do not destroy soft tissue and sometimes it is preserved too.

11. Process of fossilization

12. Fossils allow us to learn about extinct species
It is obviously possible to determine a lot of information about the morphology of extinct species from an examination of fossils, but less obvious information can be inferred also. For example, about behavior and diet.
Fossil evidence shows that some dinosaurs nested
in groups, brooded their eggs and young and
protected them from predators.

13. Fossils provide clues about behavior
Live birth in plesiosaurs
Plesiosaur being born.
Predation
Trackways provide information about
social behavior in sauropods.

14. Scanning electron microscopy provides evidence of cellular structure
Structure of melanosomes suggests striking plumage in dinosaurs.

15. Cat scans help determine function of hadrosaur crests
Crest was connected to nasal cavity and sound generated by blowing air through the cavity where it resonated.
Based on the shape of different species ear bones we can tell that individual species were tuned to the same frequency sounds as were produced by the crest.

17. Biomarkers
Some evidence of life found in rocks is not fossils, but the presence of molecules (biomarkers) that can only have been produced by living organisms.
For example, 1.64 bya rocks from Australia contain okenane, a molecule derived from okenone, whose only known source is purple sulfur bacteria which use the molecule in photosynthesis.

18. Biomarkers reveal traces of life
Biomarker: distinctive molecules only produced through biological activity
Enzymatic reaction only carried out by purple sulphur bacteria.
These bacteria live only in low oxygen, high sulfur environments.

19. Carbon isotopic signatures used to infer diet of early hominins
C4 plants (grasses) have lower C13 levels than C3 plants (e.g. bushes, trees).
C13/C14 ratio can be used to infer types of plants eaten by looking at the ratio of carbon isotopes in tooth enamel of animals.
Hominins have ratios intermediate between grazers and browsers suggesting they ate a mix of plants (and animals that ate those plants).

21. Occasionally soft tissues fossilize
The Burgess shale in British Colombia preserves a snapshot of a community of organisms 505 million years ago.
~65,000 specimens of ~93 species have been collected.

23. Burgess Shale fossils
The Burgess Shale fossils date from the so-called “Cambrian Explosion” and include a diverse array in invertebrate animals.
The Cambrian Explosion was called an “explosion” because it appeared at first that a diversity of complex multicellular animals appeared suddenly in the fossil record.
However, there is now plenty of evidence of life on earth and of multicellular organisms well before this period.

24. Earliest signs of life Oldest evidence of life dates to 3.7 bya
Carbon contained in rocks is alleged to be biological in origin, but that claim is controversial
Oldest stromatolite (bacteria) fossils date to 3.45 bya and microorganisms still dominate the planet today.
Top: Living stromatolites
Bottom: Fossil stromatolites

25. How do early organisms fit in the tree of life?
Earliest fossil Eukarya: ~1.8 bya
Earliest fossil bacteria: potentially 3.45 byo; abundant by ~2.6 bya, corresponding to rise in oxygen
Earliest fossil Archaea: ~3.5 bya

26. Timeline of life on Earth
Earliest fossils of Domain Archaea ~ 3.5 bya. These are the Archaebacteria and are extremophiles living today in e.g. hot springs, salt lakes and other challenging environments.
Earliest fossils of Domain Bacteria ~ 3.45 bya. The true bacteria. Ubiquitous on Earth. Their production of oxygen altered the planet.
Earliest fossils of Domain Eukarya ~ 1.8 bya. Eukaryotes. Much bigger more complex cells than the bacteria and archaea. Cell has a nucleus and membrane-bound organelles.

27. Oldest fossils of multicellular life date back 2.1 billion years
Unclear where they fit in the tree of life as they bear little resemblance to anything living today. Not clear if they were bacteria, archaea or eukarya.

28. Origins of multicellularity
Clearly there were multiple independent origins of multicellularity because fungi, plants and animals all include both multicellular and unicellular representatives.

29. Eukaryotic multicelluar life
Earliest fossils of filamentous algae date to 1.6 bya
Red algae: 1.2 bya
Green algae: 750 mya
Red algae fossil; 1.2 bya

30. The dawn of animals Earliest animal life strongly resembles sponges
Oldest fossils 650 myo
Biomarkers (a cholesterol-like molecule only made by sponges) has been identified in 635mya old rocks

31. Ediacaran fauna
A diverse and unique collection of animals dominated the oceans from 575 – 535 mya. Most are hard to place taxonomically as they look very strange.

32. Evolution of Ediacaran fauna

33. Diversification of animals
Only a fraction of Ediacaran fauna share traits with existing lineages and almost all were extinct within 40 million years
Most existing lineages are found for the first time in the fossil record during the Cambrian period (~ mya), including our own lineage, the chordates.

34. Chordates emerged during early Cambrian
Dunkleosteus (380 mya) a 6m
long predator.
Haikouichthys (above left) early chordate from the Cambrian

35. Transition from ocean to land was a major event in evolution
Prokaryotes colonized terrestrial environments first
Fossils date to 2.6 bya. For about 2 billion years only prokaryotes were found on land.
Terrestrial animals, plants, and fungi, appeared much later.
All faced similar challenges surviving on land. Had to avoid drying out, had to support themselves out of water, had to be able to reproduce out of water. And, for animals, had to be able to move and breathe on land.

36. First terrestrial plant and fungal life
Oldest terrestrial plant fossils are 475 myo
Early plants resembled mosses and liverworts
Large forest ecosystems developed within 100 million years (after the evolution of the vascular system).
Fungi appear ~ 400 myo
Associated with plants

37. First terrestrial animal life
Invertebrate trackways date to 480 mya
Probably relatives of insects and spiders
Not clear whether they lived on land permanently. Many modern invertebrates (e.g. crabs) live in and out of water.
Oldest fossil of a fully terrestrial animal (a millipede) dates to 428 mya.

38. First terrestrial vertebrates
Oldest track ways of terrestrial vertebrates date to 390 mya
Oldest fossils of tetrapods date to 370 mya

39. Familiar forms of life did not emerge until recently
350 million years ago many currently existing lineages had yet to evolve:
Mammals (~ 150 mya)
Birds: ~150 mya
Descendants of dinosaurs
Flowering plants: ~132 mya
Grasses did not diversify until ~20 mya
Insects: emerged ~400 mya but most current lineages appear much later
Teleost fish (early Triassic ~ 240 mya)

40. Evolution of mammals
Mammals evolved from a group called the synapsids. These included the sail-backed pelycosaurs (right)
They were the dominant vertebrates from about 300 mya to 210 mya when the Permian extinction wiped most of them out.
First mammals emerged 150 mya during the age of the dinosaurs.

41. Diversification of mammals
Mammals diversified after dinosaurs went extinct (~65 mya) in the Cretaceous extinction.
Whales, bats, and primates all emerged around 50 mya

42. Oldest modern human fossils are ~200,000 years old.