Summary
Summary History of Life Notes
- Module
- Life on Earth (GS10720)
- Institution
- Aberystwyth University (UWA)
Important Book notes from History of Life
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Life on Earth NotesOrigin of Life
The fact of observation is that there is no evidence of life, let alone evidence of intelligence or civilization,
anywhere in the universe except on our planet, Earth (for example, Smith 2011).
This fact comes in the face of strenuous efforts by science fiction writers, tabloid magazines, movie directors,
and NASA publicists to persuade us otherwise.
However, we have to face up to its implications. Most important, it implies (but does not prove) that Earth’s life
evolved here on Earth. How difficult would that have been?
We can test the idea that life evolved here on Earth, from non-living chemicals, by observation and experiment.
Geologists and astronomers look for evidence from the Earth, Moon, and other planets to reconstruct conditions
in the early solar system.
Chemists and biochemists determine how complex organic molecules could have formed in such environments.
Geologists try to find out when life appeared on Earth, and biologists design experiments to test whether these
facts fit with ideas of the evolution of life from non-living chemicals.
Complex organic molecules have been found in interstellar space, in the dust clouds around newly forming stars,
on comets and asteroids and interplanetary dust, and on the meteorites that hit Earth from time to time.
These compounds form naturally in space, generated as gas clouds, dust particles, and cometary and meteorite
surfaces are bathed in cosmic and stellar radiation.
Laboratory experiments designed to mimic such conditions in space have yielded organic molecules. Probably
any solid surface near any star in the universe received organic molecules at some point in its history (Ciesla
and Sandford 2012).
Analyses of meteorites that have hit the Earth show they were carrying many of the basic organic molecules
needed in the evolution of life.
But life as we know it is not just made of organic compounds: life consists of cells, composed mostly of liquid
water that is vital to life.
It is almost impossible to imagine the formation of any kind of water-laden cell in outer space: that can only
happen on a planet that had oceans and therefore an atmosphere.
Planets have organic compounds delivered to them from space, especially from comets or meteorites, but this
process by itself is unlikely to lead to the evolution of life.
For example, organic molecules must have been delivered everywhere in the solar system, including Mercury,
Mars, Venus, and the Moon, only to be destroyed by inhospitable conditions on those lifeless planets.
, Life exists in cells.
The simplest cell alive today is very complex: after all, its ancestors have evolved through many billions of
generations. We must try to strip away these complexities as we wonder what the first living cell might have
looked like and how it worked.
A living thing has several properties: it has organized structure, and the capacity to reproduce (replicate itself),
and to store information; and it has behaviour and energy flow (metabolism). Mineral crystals have the first two
but not the last two.
A living thing has a boundary that separates it from the environment. It operates its own chemical reactions, and
if it did not have a boundary those reactions would be unable to work, they would be diluted by outside water, or
compromised by outside contaminants.
So, a living “cell” has some sort of protective membrane around it. A cell, like a computer, has hardware,
software, and a protective case, all working well together.
The case, or cell membrane, is made from molecules called lipids. The software that contains the information for
running a cell is coded on nucleic acids (DNA and RNA), which use a four-character code rather than the two-
character code (0 and 1) that our computers all use.
The hardware consists largely of proteins, long molecules made from strings of amino acids.
All those components had to become parts of a functioning organism. A living thing can grow, and it can
replicate, that is, it can make another structure just like itself.
Both processes require complex chemistry. Growth and replication use materials that must be brought in from
outside, through the cell wall.
A living thing interacts with its environment in an active way: it has behaviour.
The simplest behaviour is the chemical flow of substances in and out of the cell, which can be turned on and off.
The chemical flow will change the immediate environment, and the presence or absence of the desired
chemicals will decide whether the cell turns the flow on or off. Temperature and other outside conditions also
affect the behaviour of even the simplest cell.
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