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

Assigned reading Assigned reading: Chapter 3 of your text book

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.

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.

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.

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.

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.

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

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.

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.

Process of fossilization

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.

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

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

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.

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.

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.

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).

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.

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.

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

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

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.

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.

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

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

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

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.

Evolution of Ediacaran fauna

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 (~540-485 mya), including our own lineage, the chordates.

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

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.

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

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.

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

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)

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.

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

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