LECTURE 8: Patterns and Processes in Life History,

Conditions on early Earth promoting origin of life

Atmosphere containing simple chemicals (methane, ammonia, water, hydrogen) where energy sources drove reactions producing organic molecules.

Miller-Urey experiment

Demonstrated that organic molecules, including amino acids, could form abiotically under simulated early Earth conditions using electricity as an energy source.

Abiotic

Occurring without living organisms; describes non-biological processes that produced early organic building blocks.

Role of clay surfaces in origin of life

Provided surfaces and mineral conditions that helped simple molecules polymerize into larger molecules like RNA using sunlight.

RNA world hypothesis

Proposes that self-replicating RNA preceded DNA, enabling natural selection to favor stable, efficiently replicating molecular structures.

Importance of self-replication in early life

Allowed molecules to copy themselves and become more abundant, enabling natural selection to act on structural variations.

Contribution of membranes to early cell formation

Phospholipid bilayers naturally self-assembled to enclose self-replicating molecules, producing early protocells.

Protocell

A membrane-bound structure containing self-replicating molecules that served as an intermediate step between nonliving chemistry and early cells.

Proposed sequence for origin of simple cells

Simple chemicals $\rightarrow$ organic molecules $\rightarrow$ polymers $\rightarrow$ self-replicating RNA $\rightarrow$ phospholipid membrane enclosure $\rightarrow$ protocells/early cells.

Characteristics of the earliest cells

Prokaryotic organisms that first appeared approximately $4\,\text{billion years ago}$.

Fossil record

Physical evidence of past organisms documenting structural changes in life throughout Earth's history.

Reasons for fossil record incompleteness

Fossilization is rare and heavily biased toward abundant species, specific habitats, certain time periods, and organisms with hard parts.

Relative dating

Method of estimating fossil age based on position in rock strata, where lower layers are generally older than upper layers.

Absolute dating

Method of determining the numerical age of fossils or rocks using the decay rates of radioactive isotopes.

Mechanism of radioactive dating

Measures the remaining ratio of decaying radioactive isotopes, which break down at known, constant rates.

Carbon-14 dating

Measures the ratio of carbon-14 to carbon-12 in organic remains, relying on decay into nitrogen-14 with a half-life of $5,730\,\text{years}$.

Fossil age and carbon-14 content

Younger fossils contain higher ratios of carbon-14 because less time has elapsed for radioactive decay to occur.

Stromatolites

Rock-like structures formed by sediment trapped in layers of cyanobacteria, representing some of the oldest fossil evidence of life.

Major sequence of events in life's history

First prokaryotes $\rightarrow$ evolution of photosynthesis $\rightarrow$ atmospheric oxygen rise $\rightarrow$ first eukaryotes $\rightarrow$ multicellularity $\rightarrow$ Cambrian diversification $\rightarrow$ land colonization.

Atmospheric conditions for the earliest organisms

Lacked oxygen (anaerobic), requiring early cells to obtain carbon and energy from organic molecules.

Oxygen Revolution

Historical period when widespread oxygenic photosynthesis caused $O_2$ to accumulate in Earth's atmosphere, permanently altering its composition.

Atmospheric effect of photosynthesis

Consumed $CO_2$ and released $O_2$, decreasing atmospheric carbon dioxide levels while accumulating oxygen.

Timeline for origin of eukaryotic cells

First appeared in the fossil record approximately $2.1\,\text{billion years ago}$.

Timeline for origin of multicellularity

Began around $1.5\,\text{billion years ago}$, with fossilized multicellular algae dating to $1.2\,\text{billion years ago}$.

Cambrian Explosion

Rapid diversification of animal life $535\text{--}525\,\text{million years ago}$, giving rise to modern animal phyla in the fossil record.

Timeline for land colonization

Fungi, plants, and animals began moving onto land around $500\,\text{million years ago}$.

Continental drift

The gradual movement of Earth's tectonic continents across the underlying mantle over geological time.

Evolutionary effect of Pangaea's breakup

Geographically isolated populations, driving allopatric speciation as landmasses separated.

Mass extinction

A rapid event resulting in the loss of a large percentage of Earth's total species diversity.

Permian mass extinction

Event $252\,\text{MYA}$ that eliminated roughly $96\%$ of marine animal species, linked to extreme volcanism, global warming, and ocean anoxia.

Cretaceous mass extinction

Event $65.5\,\text{MYA}$ caused by a meteorite impact that eliminated about $75\%$ of species, including non-avian dinosaurs.

Adaptive radiation

The evolution of diverse, specifically adapted species from a common ancestor when novel ecological niches open.

Recovery timeframe for biodiversity post-mass extinction

Requires approximately $5\text{--}100\,\text{million years}$ for species diversity to recover.