Chapter 25: The History of life on Earth

Geological events that alter environments
change the course of biological evolution; conversely life changes the planet that it inhabits
Most biologists now think that it is at least a credible hypothesis
that chemical and physical processes on early Earth produced very simple cells through a sequence of stages
There were four main stages in the evolution of life (hypothesis)
Abiotic synthesis of small organic molecules
Joining of these molecules into macromolecules
Packaging of molecules into protocells
Origin of self replicating molecules
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Earth formed about 4.6 billion years ago
along with the rest of the solar system; the first life evolved as early as billion years ago.
Earth’s early atmosphere
contained water vapor and many chemicals releases by volcanic eruptions
Oparin and Haldane hypothesis earths early atmosphere was reducing (didn’t have any oxygen, only nitrogen, hydrogen, sulfuric acid etc) and oceans were a solution of organic molecules
Laboratory experiments simulating an early Earth atmosphere- Miller and Urey
It showed that organic molecules (like amino acids ) could be produced from inorganic molecules but the existence of such an atmosphere on early Earth is unlikely
Some of the organic compounds from which the first life on earth arose
was from outer space carbon compounds found on meteorites
The possibility that life
is not restricted to earth is becoming more accessible to scientific testing
Small organic molecules
polymerize when they are concentrated on hot sand, clay or rock
Replication and metabolism are key properties of life an may have appeared together in protocells
Protocells may have formed from vesicles with a membrane like structure
In water lipids and other organic molecules cam spontaneously form vesicles with a lipid bilayer
Adding clay can increase the rate of vesicle formation
Vesicles exhibit simple reproduction and metabolism and maintain internal chemical environment
The first genetic material
was probably RNA not DNA
WHhy RNA and not DNA?
called ribozymes have been found to catalyze many differenct reactions including self splicing
making complementary copies of short streteches of their own sequences or other short pieces of RNA
How RNA developed
the main explanation for the lack of continuing abiotic origin of life on Earth today is that
our oxidizing atmosphere is not conducive to the spontaneous formation of complex molecules
One can view the chronology of major episodes that shaped life as a
phylogenetic tree
The key events
1-1st cell
2- photosynthesis
3- aerobic respiration
4- eukaryotes
5- particularity
6- Cambrian explosion (535-525 million years ago)
7- Invasion of land
The analogy of a clock
can be used to place major events in the earth’s history in the context of the geological record
Hadean (origin of earth)
Archaean (Oldest fossils of cells (prokaryotes) Appear)
Proterozoic (Oldest fossils of eukaryotes appear)
Paleozoic- The Cambrian Explosion; first tetrapods and insects appear,

Mesozoic- origins of mammals; dinosaurs evolved and radiate. (Amphibians, reptiles),

Cenozoic- Humans appear; major radiation of pollution insects (Mammals and angiosperms)

The oldest known fossils are stromatolites,
rocks formed by the accumulation of sedimentary layers on bacterial mats
Stromatolites -date back
3.5 billions years ago
Prokaryotes were
earth’s sole inhabitants for more than 1.5 billion years
Electron transport system of a variety of types
were essential to early life
Have some aspects that possibly precede life itself
The earliest types of photosynthesis
did not produce oxygen
Oxygenic photosynthesis
probably evolved around 3.5 billion years ago in cyanobacteria (oxygen revolution)
When oxygen began to accumulate in the atmospheres about 2.7 billion years ago
it posed a challenge for life
The oldest fossils of eukaryotes cells
date back 2.1 billion years
The theory of endosymbiosis
Proposed that mitochondria and plastids were formerly small prokayotrs living within larger host cells
probably gained entry to the host cells as undigested prey or internal parasites
Key evidence supporting an endosymbiotic origin of mitochondria and plastids
Inner membranes are similar to plasma membranes of prokaryotes
Division and DNA structure is similar in these organelles and some proykaryotes
Theses organelles transcribe and translate their own DNA
Their ribosomes are more similar to prokaryotic than eukaryotic ribosomes
Molecular clocks
date the common ancestor of multicellular eukaryotes to 1.5 billion years
The oldest known fossils of eukaryotes
are of relatively small algae that lived about 1.2 billion years ago
Larger organisms do not appear in the fossil record
until several hundred million years later
Chinese paleontologists recently described 570 million year old fossils that are probably animal embryos
Most of the major phyla of animals
appear suddenly in the fossil record that was laid down the first 20 million years it the Cambrian period
The fossil record
provides a glimpse into he evolution of life on Earth. It shows that there have been remarkable changes in the kinds of organisms that have been abundant at different times
How do body characteristics affect the likelihood of fossilization? `
If an organism has a soft body, it is less likely to be fossilized; a hardshell is not crushed easily and does not decompose quickly
How does habitat location affect the likelihood of fossilisation
Organisms that live in an area with a low oxygen content are more likely to fossilize, organisms that live near a beach are more likely to fossilize; organisms that live in an area where sediments are actively being deposited are more likely to fossilize
The oldest fossils usually
are found in the deepest strata
Radiometric dating
allows us to indirectly date fossils up to billion of years old based on minerals in surrounding volcanic strata
The geologic record
evidence in the fossil record of large scaled extinction and or enormous radiation of many diverse new life forms defines the main boundaries of the geologic record
Precambrian Time
Began 4600 million years ago; the first prokaryotic cells and cyanobacteria appeared as well as the first animals
Paleozoic Era (Cambrian Period)
Began 542 million years ago; the first plants, jawless fish, bony fish. Amphibians became dominant vertebrate life form and the first reptiles and seed plants appeared . By the end many marine and terrestrial organisms became extinct.
Mesozoic Era
251-65 million years ago ; reptiles radited out as dinosaurs to dominate the landscape, the first birds and mammals originated. flowering plants appeared and many organisms including most dinosaurs became extinct.
Cenozoic Era
Followed the mesozoic and continues through modern times. Angiosperms evolved to dominate the landscape; primate groups evolved leading to the origin of apelike human ancestors and finally humans.
The atmosphere of early earth probably contained no oxygen until the emergence of organisms that
used water as an electron source from photosynthesis; oxygen began to accumulate in the atmosphere due to the action of photosynthetic cyanobacteria
Adaptive radiations involved
the splitting of a single lineage into multiple species each adapted to a specific set of ecological demands
Convergent evolution
occurs when selective pressures produced by similar conditions (such as habitats) favor the evolution of similar solution
Hox genes are thought to play an important role int he development of different morphologies because
they provide positional information in the embryo

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