Lecture 12: The History of Life

Covers Chapter 18

Think of it as a story

• Early earth had little oxygen (O2 does not promote the creation of complex molecules)

• Mostly methane, ammonia, hydrogen and water• Some molecules could exist in this environment• These small molecules became more plentiful and

common• Accumulation of small organic molecules formed

a pool of building blocks

Miller and Urey simulated this O2 poor environment

• Clay and other surfaces were a place where small molecules could combine to form larger ones

• Environment contributed to this process:– Low temp– Low pressure– UV light– Electrical discharge

First complex molecule

• RNA may have been the first self-reproducing complex molecule (called a ribozyme)

• This molecule became contained within a membrane (started as a vesicle, evolved into modern phospholipid membrane)

• These early membranes could absorb material from the outside, grow and divide (and pass on copies of the ribozyme)

Ribozyme

Formation of the earth

• Earth formed 4.5 billion years ago.

• How do we know? Radiometric dating of rock (also used with fossils to determine age)

• Historians describe the formation of the earth in eras

Eras

Eras

First organisms

• First organisms were single-celled procaryotes.– Lacked membrane bound nucleus– Obtained nutrients and energy by absorbing

organic molecules from the environment

Ability to synthesize energy & appearance of oxygen*

• Some procaryotes evolved the ability to use sunlight for synthesis on complex, high-energy molecules (ATP)

• Water became the source of hydrogen in photosynthesis

• This water-based photosynthesis released oxygen gas as a by-product

• Oxygen began accumulating in the atmosphere

Ability to utilize oxygen

• Chemical analysis of rocks suggests that significant levels of oxygen appeared 2.3 billion years ago.

• Organisms evolved cellular mechanisms that allowed them to use oxygen in metabolism (cell respiration)

Creation of a cell

• Some procaryotes developed the ability to absorb other procaryotes (predation)

• Then came the ability to compartmentalize functions inside the cell (creation of organelles) PROOF: many organelles have their own DNA

• This led to first eucaryotes (engulfed procaryotes became primitive nuclei) 1.2 billion years ago

Multicellular organisms

• Larger procaryotes/newly formed eucaryotes could move faster, get prey and evade predators

• But larger procaryotes had more of a problem capturing enough energy and nutrients

• Therefore, having numerous cells would give them more ability to generate energy (each cell can generate energy…more cells, more energy)

First “animals”

• Earliest fossils of animals are invertebrates (600 million years ago)

• Sponges, jellyfish, mollusks, worms, arthropods

• Cambrian period: explosion of animal diversity

Body improvements

• Predation favored evolution of mobility and the senses

• Skeletons improved mobility and offered protection (started with exoskeleton, then fish developed the first internal skeletons)

• Some fish also developed outpouching of digestive tract (later became lungs)

Life came to land*

• Land-invading organisms had a new set of problems:– Had to support their own weight– Had to learn to find water– Had to protect gametes from getting dried out

(external fertilization up to this point..female fish release eggs, male fish swim over and drop sperm)

Plants

• Algae started growing at the edge of the water…(to decrease competition from other water plants) began to grow on land at water’s edge…evolved into land plants

• They could get nutrients from soil (nitrogen and phosphorus)

• Land was free of predators in the beginning• Plants adapted to life on land

– Vascular system (roots)– Waterproof coatings on leaves to keep water in– Egg and sperm no longer required water for

fertilization (plant retains egg and sperm released as pollen that is spread by wind)

– Evolution of flowers

Plants

• Fertilized egg stays on parent plant, gets encased as a seed (which holds nutrients for growing embryo)

• Later conifers became the first plants to encase their eggs in a cone, therefore improving chances of survival for their offspring.

• Finally, flowering plants emerged from conifers. Flowers allow animals and insects to pollinate them (actually more efficient than wind pollination

First land animals: Arthropods

• Emergence of land plants encouraged animals to leave the sea for land.

• Arthropods* came out of the water first (evolving from fish that had exoskeletons-better to stand up on land)

• Ruled the land for millions of years

• *: insects, spiders, centipedes, crabs

Amphibians

• Another group of fish (Lobefin) had soft fleshy fins and an outpouching of digestive tract that eventually became lungs. (previously all animals needed to be in the water to process oxygen)

• These animals crawled out of the sea and gave rise to *amphibians.

• Could breathe oxygen but use skin as secondary respiratory surface

• *: Frogs, toads, salamanders

Reptiles

• From amphibians

• Had 3 important adaptations:– Shelled, waterproof eggs*– Scaly, water resistant skin*– Improved lungs (animal no longer needs water

to process oxygen)*

Dinosaurs & Birds

• Both from reptiles

• Dinosaurs: some reptiles kept growing

• Birds: reptile scales evolved into feathers (easier to maintain body heat with feathers)

Mammals: Rodents, Primates, Carnivores

• From reptiles• While most reptiles laid eggs, some evolved into

animals that could carry their eggs internally and feed their offspring with milk glands.

• Fossils from this time cannot completely prove this transition (can’t tell if animal had mammary glands in a fossil)

• Also, skeleton had adaptations (limbs with better mobility)

• They flourished after dinosaurs became extinct.

Hominin: humans and their relatives

• Facultative Bipeds– Sahelanthropus: (>6 million yrs old)– Ardipithecus (4 million years old)

• Bipeds– Australopithecus (4 million years old)

• First appearance of Homo: the genus we belong to– Larger brains/skulls, smaller teeth, flatter face– Tools found next to fossils– Homo Habilis: long arms, short legs– Homo ergaster: shorter arms, longer legs (2 million yrs old)– Homo erectus– Neanderthals/Cromagnon (150,000 yrs old) artwork found with fossils – Homo Sapiens

Human adaptations*

• Binocular vision to see predators (forward-facing eyes)

• Color vision• Larger brains: improved social skills and hand-eye

coordination, language, creative thought (artwork)• Grew crops• Kept domestic animals• Grasping hands, opposable thumbs