Chapter 25: The History of Life on Earth

How Life on Earth Has Changed Over Time

  • Past organisms differed significantly from those living today.
    • Fossils in the Saharan Desert show whale transition from land to sea.
  • Macroevolution: Broad evolutionary patterns above the species level.
    • Examples from the fossil record include:
      • Emergence of terrestrial vertebrates.
      • Impact of mass extinctions.
      • Origin of key adaptations like flight.

Conditions on Early Earth and the Origin of Life

  • Chemical and physical processes could produce simple cells through four stages:
    1. Abiotic synthesis of small organic molecules.
    2. Joining of these small molecules into macromolecules (monomers into polymers).
    3. Packaging of molecules into protocells: droplets with membranes maintaining internal chemistry different from the environment.
    4. Origin of self-replicating molecules.

Synthesis of Organic Compounds on Early Earth

  • Earth formed approximately 4.6 billion years ago.
  • Collisions with rocks and ice vaporized water, preventing sea formation before 4 billion years ago.
  • Early atmosphere:
    • Little oxygen.
    • Abundant water vapor and compounds from volcanic eruptions (nitrogen oxides, carbon dioxide, methane, ammonia, hydrogen).
    • Amino acids could form under conditions simulating volcanic eruption
  • Organic compounds could have been produced in deep-sea hydrothermal vents.
    • Hot water and minerals gush from beneath the Earth’s surface into the ocean.
  • Meteorites as a source of organic molecules
    • The Murchison meteorite (4.5 billion years old) contained amino acids, lipids, simple sugars, and nitrogenous bases.

Abiotic Synthesis of Macromolecules

  • Spontaneous abiotic synthesis of RNA monomers has been demonstrated in the lab.
  • RNA polymers form spontaneously when a monomer solution is dripped onto hot sand, clay, or rock.
  • Abiotically synthesized polymers could have acted as weak catalysts on early Earth.

Protocells

  • Replication and metabolism: Key properties of life that may have appeared together in protocells.
  • Protocells may have formed from fluid-filled vesicles with a membrane-like structure.
  • In water, lipids and organic molecules can spontaneously form vesicles with a lipid bilayer.
  • Vesicles can exhibit properties of life:
    1. Simple growth without dilution of contents.
    2. Reproduction.
    3. Metabolism.
    4. Maintenance of an internal environment different from surroundings.

Self-Replicating RNA

  • RNA was likely the first genetic material, not DNA.
  • RNA plays a central role in protein synthesis.
  • Ribozymes: RNA molecules that catalyze reactions.
    • Ribozymes can make complementary copies of short stretches of RNA from nucleotides, which helps increase the efficiency of replication and diversity which helps with the chances of self replicating molecules.
  • Self-replicating ribozymes have been produced through natural selection in laboratory experiments.
  • RNA molecules with different nucleotide sequences fold into different shapes.
  • Copying errors can produce shapes that enable faster replication with fewer errors.
  • The RNA molecule with the greatest replication ability leaves the most descendent molecules.
  • Researchers created a vesicle within which copying of a template strand of RNA could occur.
  • Protocells could form from vesicles that grew, split, and passed RNA to their “daughters.”
  • Natural selection could act on such protocells, favoring successful forms.
  • RNA could have provided the template for DNA assembly.
  • Double-stranded DNA is more chemically stable and can be replicated more accurately than RNA.
  • Many scientists believe RNA came before DNA, but this is still debatable.

The Fossil Record

  • The fossil record reveals changes in the history of life on Earth.
  • Based on the accumulation of fossils in sedimentary rock layers (strata).
  • Other fossils (e.g., insects in amber) provide information.
  • The fossil record shows great changes in Earth's organisms over time.
    • Many past organisms were unlike those living today.
    • Many common organisms are now extinct.
    • New groups arose from existing ones.
  • Rise and fall of dinosaurs are memorialized by fossils.
  • The fossil record is an incomplete chronicle.
    • Few organisms were preserved as fossils.
    • Many fossils were destroyed by geologic processes.
    • Only a fraction of fossils have been discovered.
  • The known fossil record is biased towards species that:
    • Existed for a long time.
    • Were abundant and widespread.
    • Had hard parts (shells, skeletons).

Dating Rocks and Fossils

  • Fossils' order in rock strata indicates the sequence of formation.
  • Relative ages can be inferred this way, but not actual ages.
  • Radiometric dating determines fossil age based on radioactive isotope decay.
  • A radioactive “parent” isotope decays to a “daughter” isotope at a characteristic rate.
  • Each isotope has a known half-life (time for 50% of the parent isotope to decay).
  • Fossils contain isotopes accumulated during their life.
  • Age is estimated based on the ratio of carbon-14 to carbon-12 isotopes.
    • Carbon-12 is stable.
    • Carbon-14 is radioactive and decays to nitrogen-14 after death.
  • Carbon isotopes can date fossils up to 75,000 years old.
  • Older fossils require radioisotopes with longer half-lives.
  • Organisms do not use radioisotopes with long half-lives to build bones or shells.
  • Older fossils are dated using radioisotopes in surrounding volcanic rock layers.

Origin of New Groups of Organisms

  • Mammals are tetrapods.
  • Mammalian features evolved gradually over time.
  • Most tetrapods have undifferentiated, single-pointed teeth.
  • Mammalian teeth are specialized:
    • Incisors: tearing
    • Canines: piercing
    • Molars: crushing and grinding

Key Events in Life’s History

  • Geologic record divided into Hadean, Archaean, Proterozoic, and Phanerozoic eons.
  • Phanerozoic eon includes Paleozoic, Mesozoic, and Cenozoic eras.
  • Boundaries between eras correspond to major extinction events.
  • Prokaryotes existed before atmospheric oxygen; single-celled eukaryotes flourished with oxygen; animals transitioned from water to land.

First Single-Celled Organisms

  • Stromatolites are layered rocks formed when prokaryotes bind sediment films.
  • Fossilized stromatolites (3.5 billion years ago) are the earliest life evidence.
  • Prokaryotes were Earth’s sole inhabitants for over 1.5 billion years.

Photosynthesis and the Oxygen Revolution

  • Most atmospheric oxygen (O_2) is biological in origin.
  • Early Earth: O_2 produced by photosynthesis reacted with dissolved iron, forming iron oxide sediments.
  • Sediments compressed into banded iron formations (red rock layers).
  • Once dissolved iron precipitated, O_2 dissolved into the water.
  • When seas and lakes saturated, O_2 entered the atmosphere.
  • O_2 accumulated gradually from 2.7 to 2.4 billion years ago.
  • After 2.4 billion years ago, atmospheric O_2 rapidly increased to 1-10% of its present level, known as the “oxygen revolution.”
  • Oxygen attacks chemical bonds, inhibiting enzymes and damaging cells.
  • Many prokaryotic groups went extinct due to the oxygen revolution.
  • Some survivors found refuge in anaerobic habitats; others adapted to use O_2 for cellular respiration.

First Eukaryotes

  • Oldest eukaryote fossils are from single-celled organisms (1.8 billion years ago).
  • Eukaryotic cells have a nucleus, membrane-bound organelles, and a cytoskeleton.
  • The cytoskeleton allows eukaryote cells to change shape and engulf other cells.
  • Eukaryotes likely originated by endosymbiosis when a prokaryotic cell engulfed a small cell that would evolve into a mitochondrion.
  • The engulfed cell is an endosymbiont, living within the host cell.
  • Endosymbiosis hypothesis: Primitive single-celled eukaryotes using oxygen were derived from bacteria with a primitive mitochondrion.
  • The mitochondrion is the ‘powerhouse of the cell.’
  • Anaerobic host cells benefited from aerobic endosymbionts as O_2 built up.
  • Over time, the host and endosymbiont became interdependent, forming a single organism.
  • All eukaryotic cells have mitochondria, but not all have plastids (chloroplasts).
  • Serial endosymbiosis: Mitochondria evolved before plastids through endosymbiotic events.
  • Both mitochondria and plastids descended from bacterial cells.
  • The original host cell is thought to be an archaean or close relative of the archaea.
  • Archaea are often called extremophiles.
  • Evidence for the endosymbiotic origin of mitochondria and plastids:
    • Some membrane proteins are homologous to bacterial membranes.
    • Replication similar to bacterial cell division.
    • Chromosome and DNA structure similar to bacteria.
    • Both transcribe and translate their own DNA.
    • Their ribosomes are similar to those of bacteria in size, RNA sequence, and antibiotic sensitivity.

Definitions of Organism Type

  • Autotrophic: Does not require organic matter (can use gases or sunlight and convert them to energy).
  • Heterotrophic: Needs to ingest organic matter (e.g., food); most animals are heterotrophic.
  • Phototrophic: Uses light.
  • Photoautotrophic: Plants, algae, or photosynthetic bacteria converting solar energy to chemical energy without ingesting organic matter.
  • Some animals are photosynthetic; others use both photosynthesis and oxygen (mixotrophic).

Cambrian Explosion

  • Many animal phyla appear suddenly in the Cambrian period (535–525 million years ago) – the Cambrian explosion.
  • Fossils of sponges, cnidarians, and molluscs appear in older rocks from the late Proterozoic.
  • Little evidence of predation appears in fossils formed prior to the Cambrian explosion.
  • Adaptations for predation (large bodies and claws) appeared within 10 million years.
  • New defensive adaptations (sharp spines and heavy body armor) appeared in prey species.
  • Animals originated about 700 million years ago and remained small for over 100 million years.
  • They diversified explosively during the Cambrian.

Colonization of Land

  • Prokaryotes lived on land 3.2 billion years ago.
  • Fungi, plants, and animals began to colonize land about 500 million years ago.
  • Adaptations for reproduction and prevention of dehydration arose when moving to land.
  • Early signs of a wax coating on leaves and a vascular system for internal transport appeared in plants by 420 million years ago.
  • Plants and fungi likely colonized land together.
  • Mutualisms between plants and fungi (mycorrhizae) are seen in the oldest fossilized plants.
  • Arthropods and tetrapods are the most widespread and diverse land animals.
  • Arthropods were among the first animals to colonize land about 450 million years ago.
  • Tetrapods evolved from lobe-finned fishes around 365 million years ago.
  • The human lineage diverged from other primates 6–7 million years ago.
  • Modern humans originated only 195,000 years ago.

Rise and Fall of Groups of Organisms

  • The rise and fall of many groups of organisms have occurred in the history of life.
  • The rise and fall of any particular group depends on speciation and extinction rates of its member species.
  • These changes are affected by processes including plate tectonics, mass extinction, and adaptive radiation.

Plate Tectonics

  • Earth’s land masses have formed a supercontinent and then broken apart three times: 1 billion, 600 million, and 250 million years ago.
  • Plate tectonics theorizes that Earth’s crust is composed of plates floating on the underlying mantle.
  • Movements in the mantle cause the plates to gradually shift in a process called continental drift.
  • Tectonic plates can drift apart, collide (forming mountains), or slide past each other (causing earthquakes).
  • For example, the Himalayan mountains formed 45 million years ago when tectonic plates collided.

Consequences of Continental Drift

  • Formation of the supercontinent Pangaea about 250 million years ago altered many habitats:
    • Ocean basins became deeper.
    • Most shallow-water habitat was destroyed.
    • The interior of the continent became colder and drier.
  • Major changes in climate occur when a continent shifts toward or away from the equator.
    • For example, Labrador, Canada, was located in the tropics 200 million years ago.
  • Organisms must adapt to the changing climate, move to a new location, or face extinction.
  • When supercontinents break apart, regions that were once connected become isolated.
  • As a result, organisms on the new continents diverge, and allopatric speciation occurs on a grand scale.
    • For example, marsupials (kangaroos) fill ecological roles in Australia analogous to those filled by eutherians on other continents.
  • The distribution of fossils and living groups reflects the historic movement of continents.
    • For example, fossils of the same species of Permian reptiles are found in Brazil and West Africa.
    • These parts of the world were joined during the Permian but are now separated by 3,000 km of ocean.

Mass Extinctions

  • The fossil record shows that most species that have ever lived are now extinct.
  • Extinction can be caused by changes to a species’ biotic or abiotic environment.
  • Mass extinctions occur when large numbers of species rapidly become extinct worldwide.

The Big Five Mass Extinction Events

  • Mass extinctions are triggered by disruptive global change.
  • Five mass extinctions have been documented in the fossil record over the past 500 million years.
  • More than half of all marine species became extinct in each event.
  • The Permian extinction (252 million years ago) divides the Paleozoic from the Mesozoic era.
  • About 96% of marine species became extinct in less than 500,000 years during this mass extinction.
  • It occurred during an extreme episode of volcanism.
    • For example, about 1.6 million km^2 in Siberia was covered with lava hundreds of meters thick.
  • Volcanic eruptions triggered catastrophic events leading to mass extinction:
    • Atmospheric CO_2 rose dramatically.
    • The global climate warmed by about 6 C^\,circ.
    • Ocean acidification reduced calcium carbonate for reef-building corals and shell-building species.
    • Nutrient enrichment of oceans caused microbial blooms, leading to anoxic conditions.
  • The Cretaceous mass extinction occurred about 66 million years ago.
  • More than 50% of marine species, many families of terrestrial plants and animals, and all dinosaurs (except birds) became extinct during this event.
  • Large-scale volcanic eruptions, prior to the meteor impact, left many species vulnerable to extinction.
  • Wildfires resulting from the impact contributed to rising CO_2 and 100,000 years of global warming.

Is a Sixth Mass Extinction Under Way?

  • The current extinction rate is estimated at 100 to 1,000 times the background rate of the fossil record.
  • It is hard to say if we are in a sixth mass extinction, due to challenges in documenting current extinctions.
  • Species losses to date have not yet reached the level of the “big five” mass extinctions, however:
    • Habitat loss, introduced species, and overharvesting are factors contributing to rapid species decline.
    • The global climate is warming, and historically, extinction rates increase with high global temperature.
  • Unless dramatic actions are taken, a sixth, human-caused mass extinction is likely to occur within the next few centuries.

Consequences of Mass Extinctions

  • It typically takes 5–10 million years for diversity to recover following a mass extinction, but rates vary.
    • For example, it took about 100 million years for marine families to recover after the Permian mass extinction.
  • Mass extinctions can change the types of organisms found in ecological communities.
    • For example, after the Permian and Cretaceous mass extinctions, the percent of marine predators increased.
  • Mass extinctions can also curtail lineages with novel and advantageous features.
    • For example, gastropods that could drill through the shells of their prey were lost in the extinction at the end of the Triassic.
    • This ability did not reappear for 120 million years!
  • By eliminating so many species, mass extinctions pave the way for adaptive radiations and the proliferation of new groups of organisms.

Adaptive Radiations

  • Adaptive radiation is a rapid period of evolutionary change where many new species arise and adapt to different ecological niches.
  • Adaptive radiations can occur in response to
    • The opening of niches following mass extinctions.
    • The evolution of novel characteristics that enable exploitation of new resources or habitats.
    • The colonization of new regions with few or weak competitors.

Worldwide Adaptive Radiations

  • Prior to 66 million years ago, the size and diversity of mammals was restricted by predation and competition from dinosaurs.
  • After the extinction of terrestrial dinosaurs, mammals underwent an adaptive radiation.
  • They diversified and filled the ecological niches left open following the mass extinction.
  • Several adaptive radiations have occurred in response to the evolution of major innovations:
    • The rise of photosynthetic prokaryotes.
    • The evolution of large predators in the Cambrian explosion.
    • The colonization of land by plants, insects, and tetrapods.
  • Adaptive radiations by plants, insects, and tetrapods followed the evolution of key adaptations for survival on land.
    • For example, the evolution of supportive stems and a water-protective coat enabled the diversification of plants on land (prevent dehydration).
  • Some groups diversified as adaptive radiations in other groups provided new food sources.
    • For example, the adaptive radiation of insects followed the diversification of the plants they ate and pollinated.

Regional Adaptive Radiations

  • The Hawaiian Islands were formed by volcanic eruptions, 3,500 km from the nearest continent.
  • Each was initially devoid of life and populated slowly by stray organisms from the mainland.
  • Multiple invasions were followed by speciation events as organisms adapted to the diverse habitats.
  • Thousands of species are unique to these islands.

Changes in Spatial Pattern

  • Changes in genes that control the placement and organization of body parts can drive evolution.
  • Homeotic genes are master regulatory genes that determine where an organism’s features will develop.
    • For example, they determine the location of a bird’s wings or the arrangement of a plant’s flower parts.
  • Hox genes, a class of homeotic genes, provide positional information in animal embryos.
  • If the location of Hox gene expression changes, the position of the corresponding body part changes.
    • For example, in crustaceans, a change in Hox gene expression produces a swimming appendage where a feeding appendage should be.

Changes in Gene Regulation

  • Many morphological changes are caused by mutations affecting developmental gene regulation.
    • For example, threespine sticklebacks in lakes have fewer spines than their marine relatives.
    • The sequence of the Pitx1 developmental gene is the same, but regulation of its expression differs between lake and marine groups.

Evolution Is Not Goal Oriented

  • Evolution is like tinkering—new forms arise by the slight modification of existing structures or developmental genes.

Evolutionary Novelties

  • Most novel biological structures evolve in many stages from simpler ancestral structures.
    • For example, complex eyes have evolved from simple photosensitive cells independently many times.
    • Such “simple” eyes are found in molluscs called limpets.
  • Exaptations are structures that evolve in one context but become co-opted for a different function.
  • Structures do not evolve in anticipation of future use; natural selection can only improve a structure in the context of its current utility.

Evolutionary Trends

  • As populations undergo natural selection, species undergo species selection.
  • Species that endure the longest and generate the most new species determine the direction of evolutionary trends.
  • Evolutionary trends do not imply an intrinsic drive toward a particular phenotype.
  • Evolution results from interactions between organisms and their current environment; if conditions change, the trend will cease or change.