Top Down & Bottom Up

ORIGIN OF LIFE: IN SEARCH OF THE SIMPLEST CELL: Modified: "There are two approaches to the 'minimal cell': the top-down and the bottom-up.
The top-down approach aims at simplifying existing small organisms, possibly arriving at a minimal genome.
The bottom-up approach aims at constructing artificial chemical supersystems that could be considered alive. No such experimental system exists yet; at least one component is always missing.

Metabolism seems to be the stepchild in the family. Basically, what most researchers in the field used to call 'metabolism' is typically the trivial outcome of need for material input of both template replication and membrane growth. This input is usually simplified to a conversion reaction from precursors to products."

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Primordial Soup : Theories

The Woodstock of Evolution -- The World Summit on Evolution (ScientificAmerican.com): Antonio Lazcano, President of the International Society for the Study of the Origins of Life and a scientist at the Universidad Autónoma de México, theorized that there were three sources for the primordial soup:

1) A reducing atmosphere from volcanic outgassing,
2) High-temperature submarine vents and fumaroles -- black smokers, and
3) Space--for example, the 4.6 billion-year-old Murchison meteorite, discovered in Australia in 1969 was loaded with amino acids, aliphatic and aromatic hydrocarbons, hydroxy acids, purines, pyrimidines, and other chemical building blocks of life.

'The evidence strongly suggests that prior to the origin of life the primitive Earth already had many different catalytic agents, polymers with sequences of nucleotides, and membrane-forming compounds,' Lazcano concluded. This prebiotic soup led to a catalytic and replicative RNA world, which led to the DNA world of today."

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The Miller-Urey experiments and beyond

In 1953, graduate student Miller set up an experiment in which he passed an electrical current (to simulate lightning discharges in the primordial atmosphere) through the gases then believed to constitute the primordial atmosphere – methane (CH4), ammonia (NH3), hydrogen (H2), and water (H2O). After a WEEK, Miller assayed the resulting chemicals and found that up to 15% of the carbon had been converted to organic compounds. Two percent of the carbon had formed amino acids, including 13 of the 21 that comprise the proteins of living cells. The smallest amino acid, glycine, was the most abundant.

It turns out that Miller’s and Urey’s guess about the composition of the primordial atmosphere was wrong, yet the Miller-Urey products and many more products (including nucleotides) have repeatedly been generated in similar experiments that more accurately replicate the early atmosphere.

The salient point is that in a very short time, a variety of life-related chemicals will generate themselves from simple molecules provided that some energy is supplied to the system. Considering the billions of years before recognizable life appeared on the entire planet, it is clear that chemical evolution could yield cellular life. In fact, the earliest discovered microfossils date from about 3 ½ billion years ago, and this is a minimum 1 billion years since the origin of the planet.

Similar processes to those above must be occurring in the energy-riddled immensity of space, because many organic compounds have been identified spectroscopically in space and have been found in meteorites.

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