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A Brief History of Life On Earth
Transcript of A Brief History of Life On Earth
(Brief) History of Life
The first Eukaryotes
ca. 1.3 Ma:
Why do we think this happened?
This kind of thinking is called
What was Early Earth like?*
Hot: Due to the formation of the Earth
Violent: Due to the formation of the Solar System
Toxic: Lots of nasty gasses in the amosphere (CO2, NH3, SO2, H2O). NO OXYGEN GAS!
The "Heavy Bombardment": Comes from being bombarded by comets & asteroids...a lot.
Something really big blasted off the moon
The atmosphere was either reducing, or neutral (the modern atmosphere is oxidizing)...favoring increasing chemical complexity.
* We Think
Make Sure You Can
How old is the Earth? Where did it come from?
How did life begin?
How have the history of life and the history of Earth influenced each other?
Explain how scientists are able to date the ages of all events discussed in this presentation.
Describe the hypothetical steps that had to occur for life to arise in the Universe.
Explain the significance of all time periods discussed in this presentation.
Describe the effects of oxygenation of the atmosphere, extinctions and adaptive radiation on the history of life.
Explain how the field of evo-devo has informed our thinking about the relationship between changes in genes and changes in forms.
The Origin of Life in 4 Steps
1. Formation of Biological Molecules
2. Accretion of proto-cells
3. Development of an information molecule
Biological molecules are complex.
Could they be created on Early Earth?
Research suggests a resounding "yes"!
Stanley Miller (1953)- simulated "Early Earth" conditions in the lab.
Created amino acids, other organic molecules
Other experiments have since demonstrated biological molecule formation in:
Asteroids (in space)
Comets (in space)
Collections of self-organized, endogenously ordered, spherical collection of lipids
) have been investigated extensively in labs.
Have a primitive metabolism
Absorb & excrete biological molecules
The "RNA World":
a hypothesis that proposes that RNA was the first information molecule.
Based on the fact that RNA has both information storage & catalytic functions.
Ribozymes: RNA enzymes that can catalyze reactions
Artificial evolution of RNA molecules
has been demonstrated in laboratory investigations
A few things to keep in mind:
1. It could have taken hundreds of millions of years to happen (we've got plenty of time).
2. Once it starts, evolution creates a positive feedback loop that leads to increasing efficiency & complexity of life.
3. Maybe it happened somewhere else, first ("
How do we know when things happened?
A Note about Ages
A “parent” isotope decays to a “daughter” isotope at a constant rate
Each isotope has a known half-life, the time required for half the parent isotope to decay, and the half-life of radioisotopes is a universal constant
The amount of particular radioisotope left = age
(within a few million years)
~15 billion years old
(based on light years of background radiation from the Big Bang)
Solar System & Earth:
~4.5 billion years old
(dating of asteroids)
Photosynthesis evolves ~500 million years after life evolves.
The Heterotroph Hypothesis
prior to photosynthesis, life ate other life (was
Evolution of Eukaryotes
Cyanobacteria: photosynthetic bacterial cells.
Stromatolites: Large colonies of cyanobacteria
Oxygenation of Earth
Any life more complex than bacteria is eukaryotic.
Eukaryotes have many compartmentalized organelles that prokaryotes do not.
Where did these organelles come from?
The availability of Oxygen in the atmosphere allows for the evolution of
aerobic cellular respiration
, a process that produces more than
the amount of ATP from breaking down biological molecules than oxygen-free (
) respiration does.
...how should this effect life's evolution?
And Away We Go!
The theory that particular organelles (mitochondria & chloroplasts) evolved from free-living bacteria which were engulfed by larger bacteria.
Chloroplasts & Mitochondria:
Reproduce independently from cell division
Have their own DNA (bacterial in structure)
DNA sequencing demonstrates bacterial origins
Have their own ribosomes (bacterial in size)
Have a double membrane
Endosymbiosis was a win-win situation.
a species of social amoeba that has unicellular and multicellular phases of its life cycle
Patterns in the Evolution of Life
After a slow start (~3.5 billion years). The biodiversity of life increases exponentially from ~550 million years ago until today.
This kind of pattern is indicative of a positive feedback relationship between evolution of complexity and time
. Why would such a relationship exist?
The Evolution of Homo
Even though the diversity of life has increased, most species (99.9%) that have ever evolved have gone
Examination of fossil evidence has show that the history of life has been punctuated by 5 particularly large extinction episodes (
the great extinctions
); during which more than 50% of Earth's species became extinct
. There is much debate about the causes.
Contrasted with extinctions, the pattern of evolution also includes many examples of the relatively fast appearance of many species.
the evolution of diversely adapted species from a common ancestor upon introduction to new environmental opportunities (i.e. when one species rapidly evolves to fill many diverse niches). Typically happens on islands (finches) or after a big extinction (mammals)
The evolution of humans is noteworthy for the effect that humans have had on the rest of the planet.
- The current geological period (from about 11,ooo years ago), wherein the actions of the human population are able to have a global impact.
Evolution Up Close
A little change can go a long way
genes are responsible for the development of specific sections of animals. Changes in the patterns of expression of these genes can lead to big changes in an animal's body plan
(like misplaced limbs)
Time sequence model of early solar system
An example of a ribozyme that manipulates other RNA molecules
The Revolution of Photosynthesis
Complexity of life increases in exponentially shorter time periods
Extinction rate vs. Number of taxonomic families over time (can you spot the 5 great extinctions?)
Artists rendering of the Chixulub impact event which is thought to have precipitated the KT extinction.
The radiation of all animal phyla
Three (of many) species that were driven to extinction by the actions of the human population:
The Passenger Pigeon
The Tasmanian Tiger
The Great Auk
Human population growth vs. number of recorded extinctions since 1400 ad
The progression of the hominid lineage from divergence with Chimpanzees to
gene expression correlated to body segments in a fruit fly
Differences in 1
gene expression in insects and crustaceans of the arthropod phylum
The expression pattern of a specific
gene (purple stain) is responsible for limbless segments in chordates
Earth formed about 4.6 billion years ago
, along with the rest of the solar system.
Its early atmosphere likely contained water vapor and chemicals released by volcanic eruptions (nitrogen, nitrogen oxides, carbon dioxide, methane, ammonia, hydrogen, hydrogen sulfide)
Instead of forming in the atmosphere,
the first organic compounds may have been synthesized near submerged volcanoes and deep-sea vents
Amino acids have also been found in meteorites; small organic molecules polymerize when they are concentrated on hot sand, clay, or rock
The Fossil Record
The fossil record reveals changes in the history of life on earth. Sedimentary rocks are deposited into layers called strata and are the richest source of fossils
The geologic record is divided into the Archaean, the Proterozoic, and the Phanerozoic eons
The Phanerozoic encompasses multicellular eukaryotic life, and is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic
Major boundaries between geological divisions correspond to extinction events in the fossil record
The oldest known fossils are
, rock-like structures composed of many layers of bacteria and sediment, and date back 3.5 billion years ago
Prokaryotes were Earth’s sole inhabitants from 3.5 to about 2.1 billion years ago
Most atmospheric oxygen (O2) is of biological origin.
O2 produced by oxygenic photosynthesis reacted with dissolved iron and precipitated out to form banded iron formations. The source of this O2 was likely bacteria similar to modern cyanobacteria
2.7 billion years ago, O2 began accumulating in the atmosphere
and rusting iron-rich terrestrial rocks.
This “oxygen revolution” from 2.7 to 2.2 billion years ago:
Posed a challenge for life
Provided opportunity to gain energy from light
Allowed organisms to exploit new ecosystems
ca. 2100 Ma:
The prokaryotic ancestors of mitochondria and plastids probably gained entry to the host cell as undigested prey or internal parasites. In the process of becoming more interdependent, the host and endosymbionts would have become a single organism
The evolution of eukaryotic cells allowed for a greater range of unicellular forms. A second wave of diversification occurred when multicellularity evolved and gave rise to algae, plants, fungi, and animals
(535 to 525 million years ago) marks an important point in the history of life on Earth; it is the time when most of the major groups of animals first appear in the fossil record. This event is sometimes called the "
," because of the relatively short time over which this diversity of forms appears. The Cambrian explosion also provides the first evidence of predator-prey interactions
Consequences of Continental Drift
Formation of the supercontinent
about 250 million years ago had many effects:
A reduction in shallow water habitat
A colder and drier climate inland
Changes in climate as continents moved toward and away from the poles
Changes in ocean circulation patterns leading to global cooling
The break-up of Pangaea lead to
, and the current distribution of fossils reflects the movement of continental drift.
For example, the similarity of fossils in parts of South America and Africa is consistent with the idea that these continents were formerly attached
defines the boundary between the Paleozoic and Mesozoic eras. It occurred in less than 5 million years, and caused the extinction of about 96% of marine animal species. This event might have been caused by volcanism, which lead to global warming, and a decrease in oceanic oxygen
Cretaceous mass extinction
65.5 million years ago separates the Mesozoic from the Cenozoic. Organisms that went extinct include about half of all marine species and many terrestrial plants and animals, including most dinosaurs
Consequences of Mass Extinctions
Scientists estimate that the current rate of extinction is
100 to 1,000 times
the typical background rate
Data suggest that a sixth human-caused mass extinction is likely to occur unless dramatic action is taken
Mass extinction can alter ecological communities and the niches available to organisms, and it can take from 5 to 100 million years for diversity to recover following a mass extinction. However, mass extinction can pave the way for adaptive radiations
Mammals underwent an adaptive radiation after the extinction of terrestrial dinosaurs
The disappearance of dinosaurs (except birds) allowed for the expansion of mammals in diversity and size. Other notable radiations include photosynthetic prokaryotes, large predators in the Cambrian, land plants, insects, and tetrapods
Adaptive radiations can occur when organisms colonize new environments with little competition.
For example, the Hawaiian Islands are one of the world’s great showcases of adaptive radiation
Genes that program development control the rate, timing, and spatial pattern of changes in an organism’s form as it develops into an adult
is an evolutionary change in the rate or timing of developmental events, which means it can have a significant impact on body shape. The contrasting shapes of human and chimpanzee skulls are the result of small changes in relative growth rates
Heterochrony can alter the timing of reproductive development relative to the development of nonreproductive organs
, the rate of reproductive development accelerates compared with somatic development. The sexually mature species may therefore retain body features that were juvenile structures in an ancestral species
Substantial evolutionary change can also result from alterations in genes that control the placement and organization of body parts.
determine such basic features as where wings and legs will develop on a bird or how a flower’s parts are arranged
Evolution of vertebrates from invertebrate animals was associated with alterations in Hox genes
Two duplications of Hox genes have occurred in the vertebrate lineage; these duplications may have been important in the evolution of new vertebrate characteristics
Changes in developmental genes can result in new morphological forms. These new forms likely come from gene duplication events that produce new developmental genes
A possible mechanism for the evolution of six-legged insects from a many-legged crustacean ancestor has been demonstrated in lab experiments. Specific changes in the Ubx gene have been identified that can “turn off” leg development
Changes in the form of organisms may be caused more often by changes in the regulation of developmental genes instead of changes in their sequence.
For example three-spine sticklebacks in lakes have fewer spines than their marine relatives. The gene sequence remains the same, but
the regulation of gene expression is different
in the two groups of fish
Most novel biological structures evolve in many stages from previously existing structures.
For example, complex eyes have evolved from simple photosensitive cells independently many times
are structures that evolve in one context but become co-opted for a different function.
For example, feathers initially evolved for heat regulation, were co-opted for display, and later co-opted for use in bird flight
Natural selection can only improve a structure in the context of its current utility
According to the
species selection model
, trends may result when species with certain characteristics endure longer and speciate more often than those with other characteristics. However, the appearance of an evolutionary trend does
imply that there is some intrinsic drive toward a particular phenotype
Evolution on Earth in 60 Seconds