Properties of lifeEdit
- Organization: Being structurally composed of one or more cells, which are the basic units of life.
- prokaryote: no nucleus
- eukaryote: membrane bound nucleus.
- Sensitivity: respond to stimuli.
- Energy Processing
- Growth and Development
- hereditary mechanisms to make more of self; DNA based.
- Regulation, including homeostasis.
Origin of life: 3 hypothesesEdit
- Extraterrestrial origin (panspermia): meteor, comet borne from elsewhere in universe
- evidence of amino acids and other organic material in space (but often both D & L forms)
- questionable bacterial fossils in Martian rock
-However, this would imply that some other origin of life was likely because it would have had to happen elsewhere before it could be transported here, and the only difference would be that life did not originate on Earth.
- Spontaneous origin on earth: primitive self-replicating macromolecules acted upon by natural selection ((macro)Evolution is one example of this)
-This is often criticized for the improbability of life being produced by a chemical reaction caused by lightning, along with the ability of any produced DNA to be in a sequence that could reproduce a working life model. It is also attacked for religious reasons, as it bypasses things like the idea of a supreme being directly creating humans. It also seems unlikely to some that such huge changes are possible in evolution without evidence of a credible "in-between” stage. Many of the stages of man are disputed due to their somewhat shaky grounds. For example, bones from other animals or species have been mistaken for humanoid while complete skeletons have been put together from a limited number of bones.
- Special creation: religious explanations (Intelligent Design is one popular example of this.) These explanations contend that life was created by God (or perhaps some other Intelligent Designer).
- Proponents of Intelligent design suggest that the vast complexity of life could only have been intentionally designed while other creationists cite biblical support.
-This is often attacked for many of the same reasons that religion is attacked, and is often regarded as superstitious and/or unscientific.
- It is debated whether schools should teach one hypothesis over another when discussing the origin of life. However, since they are all accepted hypotheses in some respect, this wikibook will include both of them without favoring one over the other.
The early earthEdit
It is believed that the Earth was formed about 4.5 billion years ago.
- Heavy bombardment by rubble ceased about 3.8 billion years ago.
- Reducing atmosphere: much free H
- also H2O, NH3, CH4
- little, if any, free O2
- with numerous H electrons, require little energy to form organic compounds with C
- Warm oceans, estimated at 49-88 °C
- Lack of O2 and consequent ozone (O3) meant considerable UV energy
Chemical reactions on early earth
- UV and other energy sources would promote chemical reactions and formation of organic molecules
- Testable hypothesis: Miller-Urey experiment
- simulated early atmospheric conditions
- found amino acids, sugars, etc., building blocks of life
- won Nobel prize for work
- experiment showed prebiotic synthesis of biological molecules was possible
- Miller later conceded that the conditions in his experiments were not representative of what is currently thought to be those of early earth
- He also conceded that science has no answer for how amino acids could self-organize into replicating molecules and cells
- In the 50 years since Miller-Urey, significant issues and problems for biogenesis have been identified. This is a weak hypothesis at this time.
- Conclusion: Life exists, we don't know why.
Origin of cellsEdit
Cells are very small and decompose quickly after death. As such, fossils of the earliest cells do not exist. Scientists have had to form a variety of theories on how cells (and hence life) was created on Earth.
- Bubble hypothesis
- A. Oparin, J.B.S. Haldane, 1930’s
- Primary abiogenesis: life as consequence of geochemical processes
- Protobionts: isolated collections of organic material enclosed in hydrophobic bubbles
- Numerous variants: microspheres, protocells, protobionts, micelles, liposomes, coacervates
- Other surfaces for evolution of life
- deep sea thermal vents
- ice crystals
- clay surfaces
- tidal pools
The RNA world?Edit
- DNA → RNA → polypeptide (protein)
- Catalytic RNA: ribozyme
- discovered independently by Tom Cech and Sid Altman (Nobel prize)
- catalytic properties: hydrolysis, polymerization, peptide bond formation, etc.
- Self-replicating RNA molecule may have given rise to life
- consistent with numerous roles for RNA in cells as well as roles for ribonucleotides (ATP)
- relationship to bubble-like structures is uncertain
The earliest cellsEdit
- ~3.5 by
- resemble bacteria: prokaryotes
- biochemical residues
- Archaebacteria (more properly Archaea)
- extremophiles: salt, acid, alkali, heat, methanogens
- may not represent most ancient life
- cyanobacteria: photosynthesis
- atmospheric O2; limestone deposits
- chloroplasts of eukaryotes
Major steps in evolution of lifeEdit
- Prebiotic synthesis of macromolecules
- Self replication
- RNA? (primitive metabolism)
- DNA as hereditary material
- 1st cells
- Aerobic respiration
- Multicellularity (more than once)
This text is based on notes very generously donated by Dr. Paul Doerder, Ph.D., of the Cleveland State University.