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

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

Protocells

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

ONce

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

Eons

Hadean (origin of earth)

Archaean (Oldest fossils of cells (prokaryotes) Appear)

Proterozoic (Oldest fossils of eukaryotes appear)

Phanerozoic:

Eras

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