Early Diversification - Lecture 7

When did the first life evolve?

Physical evidence: geological record — footprints, fossils, rock formation

Fossil preservation — leads to bias

Stages in fossilization

  1. organism dies

  2. Natural decomposition delayed

    1. Mass sedimentation buries corpse, push oxygen out (O needed for decomposition)

    2. Dead organism drifts deep into marine envr (low O)

  3. Organic molecules replaced by mineral deposits that replicate the shape of the organism (details preserved)

Fossil formation not equal among all envr (depends on moisture, O availability, disasters for ex. are more conducive towards fossil presevration — sedimentation??)

Not all animals equally likely to fossilize

  • Hard to ID from small parts

Fossils found eclusively in sedimentary rock

  • Sedimentary rock can be lost in metamorphosis, and recycled into mantle at a convergent boundary

Chemical evidence — carbon isotope ratio

  • metabolic processes used by life can leave chemical traces

  • photosynthetic organisms

  • C has multiple isotopes in nature C14 used to date

  • C12, C13 are very stable, and do not break down over time

Photosynthesis: more likely to capture CO2 containing lighter isotope

If rock contains more C12 than C13 —> suggests that in rock formation there is an organism that fixed this

But C isotope ratios cannot provide evidence for earlier life forms, as photosynthesis evolved later

  • Carbon in rock fixed by photosynthesis??

Genetic evidence:

  • estimate divergence of 2 speces

  • calculate and compare mutation rates in DNA over time

    • only works in comparing regions of DNA w/ a known slow mutation rate

    • only really possible with mitochondrial DNA (rate of survivable mutation in mitochondrial DNA is very low)

    • But again, the mitochondrai did not evolve until later

DNA can switch between individuals (prokaryotes)

Oldest scientifically accepted evidence of fossils: Stromatalites

  • colonies of bacteria

  • photosynthetic bacteria

  • as they grow, they lay down minerals on the edge of the colonies so they can maximize their photosynthetic rate

  • Still exist today

Stromatalites are colonial, photosynthetic which are both highly evolved factors

  • So, we know they are not the earliest forms of life

First life

  • primitive binary fission

  • RNA as genetic material

Major evolutionary developments in early life

Eukaryotic cell:

  • ancestral archea cell engulfed a bacteria cell

  • instead of digesting bacteria, teh bacteria remained alive and functional, fusing with the archaea which created a hybrid

  • Double organelles and DNA necessary for life

  • Redundnat genes

    • Strongly correlated to evolutionary leaps

Aerobic respiration

~2.4-2.1 GYA chemical composition of eath completely changed

  • oxidizing

  • Great Oxygenation event

  • all life forms that could not find a way to de-toxify O went extinct

Anaerobic species are only able to survive in low O areas

Chemical evolution of survivors of great event > aerobe species

  • purpose of antioxidants

Much higher rates of energy used in supporting life with O

  • anaerobic : 2 ATP

  • Aerobic: 36-38 ATP

Multicellular life emerges

  • Functional community

  • stick a bunch of single cells together and maximize size

  • increase competition

  • differentiation in roles of cells

Single cell can only get os big

  • how much of surface area is exposed to envr, larger interiorm more it needs to suck thru membrane, but the membrane isnt increasing with the rate at which the interior is growing

colonial hypothesis — purely theoretical

  • Intermediate between unicellular and multicellular

  • errors in cell dividiosn

    • 2 daughter cells do not properly separate

    • recognize each other as the same cell, no competition

    • Get enough of them in same area, cooperative interactions between single cells

Multicellular radiation

  • Increased atm. O2

  • climate change

  • snowball earth, in conjunction with rise of O2

  • slushball earth

  • LUCA evolves during snowball/slushball / molec. clock evidence

    • genetic bottlenecks

Ediacaran period

  • First complex and large multicellular life forms

  • most unique body shapes

  • most unique ecosystems

  • no hard structures

  • mainly sessile or slow moving

  • soft and squishy

  • flat or other ways to extent surface area

  • so morphologically different from modern animals

    • Difficult to decide what class of life they belong to

  • Absorption of macromolecules from envr, much like single cells

  • ocean floor of ediacaran systems

    • Microbial mats

    • build up over time

    • Believed that sessile organisms settle and graze on microbes

  • no obvious predator/prey adaptations

  • filter feeders

  • no hard mineralized structures yet