Origin of Life Pages 261-272 Modern Biology Holt, Rinehart, Winston

Biogenesis

• • Biogenesis: all living things come from other living things.

Spontaneous generation: living things come from non-living things (false statement)

Redi, Spallanzani and Pasteur

• • • • Redi set up jars with meat. The control uncovered, the experimental covered. Maggots appeared only in the control group.

Spallanzani used boiled broth, with his control group being uncovered and experimental group covered. His control group became cloudy.

Pastuer used a curve-necked flask and then a flask with the neck removed. The flask with no neck became cloudy.

All disproved spontaneous generation.

Earth’s History

• • • • Estimated age of the Earth 4.6 billion years, 700,00 times as long as the period of recorded history.

Through carbon-14 dating (radioactive dating) and studying layers of the Earth’s crust, the age was determined.

Isotopes of carbon decay with a half-life. When an organism dies, traces of carbon-14 in the body start to decay with respect to the normal carbon in the body carbon-12.

Carbon-14 has a 5715 year half-life, so after this time, half of it is decayed in a body.

Formation of Organic compounds

• • • Oparin(scientist) thought that early atmosphere of the Earth consisted of ammonia(NH3), Hydrogen gas(H2), water vapor(H2O) and compounds made of carbon and hydrogen like methane(CH4). Later was the formation of carbon dioxide(CO2) and nitrogen gas(N2).

These gases may have come together to form amino acids and macromolecules, the basic building blocks of all life.

Space debris may have also contributed to the formation of organic compounds.

From Molecule to Cell-like Structure

• • • • Simple organic chemicals may have started all life on Earth.

Ex:Microshperes are composed of many protein molecules that are organized as a membrane.

Ex: Coacervates are composed of amino acids and sugars.

Membrane bound structures may have existed on early Earth and may have had enclosed replicating molecules of RNA. Their descendants may have been the first cells.

The Origin of Heredity

• • • • DNA serves as a template for RNA which serves as a template for specific proteins.

Since RNA may assume different shapes, some RNA molecules may behave like proteins and catalyze chemical reactions.

A ribosome can act as an enzyme and may have the ability to replicate itself.

Since RNA plays a vital role in the replication of DNA, self-replicating RNA would have provided the heredity information in cells. So RNA was critical for the beginning of life.

The First Prokaryotes

• The first cells were prokaryotes, simple cells with no nuclear membrane. These would have been anaerobic (without oxygen) and heterotrophs (self feeders). They would eventually have to evolve into heterotrophs (feeds on others).

Chemosynthesis

• • Chemosynthesis: CO2 serves as a carbon source for the assembly of organic molecules. Energy is obtained from the oxidation of various inorganic substances, such as sulfur.

Some of the earliest bacteria, archaebacteria lived under harsh conditions and used chemosynthesis for energy.

Photosynthesis

• • • • Oxygen was destructive to early cell function.

When oxygen bonds, it is not as destructive. So aerobic respiration (with oxygen) occurred to prevent the destruction of organic compounds by oxygen.

Photosynthetic organisms started to evolve 3.5 billion years ago. The are related to the modern cyanobacteria.

It took billions of years for oxygen to reach today’s level. With sunlight splitting the oxygen molecule and O3 being formed in the upper atmosphere, ozone (O3) absorbed ultraviolet light, thus making an atmosphere that could sustain life.

The First Eukaryotes

• • • Eukaryotes are larger and have their DNA enclosed in a nucleus with a membrane.

Small aerobic prokaryotes started living in other larger prokaryotes which led to endosymbiosis.

The smaller aerobic prokaryotes eventually gave rise to modern mitochondria and small photosynthetic prokaryotes gave rise to chloroplasts. These two carry their own DNA and replicate independently from the rest of the cell.