Remember, the chemistry/biology of 4 billion years ago was totally different than it is today. Today, everything is facilitated by Cells :

Properties of Cells Today:

Prokaryotes

Eukaryotes-

Prokaryotes precede Eukaryotes by approximately 2 billion years. The evolutionary transition from prokaryotic cell to eukaryotic cells (those with nucleus that contain the DNA) is still not well understood. Many theories exist. One of them, the endosymbiotic theory was for decades dismissed as a relative crackpot theory but now is more widely accepted.

Making Polymers Today:

4 Billion years ago there were no cells to facilitate this. The available energy sources were

All of the above are quite inefficient compared to enzymes

Now we have a multiplying population of bacteria (prokaryotes) that has to eat to survive:

What to eat?

EVOLUTIONARY ADVANTAGE: Bacteria that could process plentiful UV light and INorganic molecules will survive as they have more "food" stocks available.

There is still a problem:

So, chlorophyll evolves to more complex forms so that it can capture and store enough UV energy to breaking the hydrogen-oxygen bond in water.

Now we have a mechanism to deliver Oxygen to the atmosphere via the aerobic photosynthesis of blue-green algae. But, the oxygen does not initially go into the earth's atmosphere. Instead we have to wait 1.5 -2 billion years to get oxygen into our atmosphere!

So we have to do all of this before we have oxygen in the atmosphere as well as an ozone layer:

So now we can crawl out on the Land and not be fried by UV radiation as the protective Ozone layer is now in place.

Life:

Evolution and Chance:

Dispersal of Parental Units by a variety of mechanisms:

Passive: (slowly alters gene pool over time)

Active: (catastrophe "overnight" change in ecosystem)

Survival of the fittest works in well-defined, slowly evolving ecosystems (like the ocean); On long time scales, the role of random catastrophe is more important

Mass Extinctions in Geological History:

Mass extinctions mean:

Timescale of extinction is crucial in identifying its cause:

Evidence from Fossil Record: 100-150 million year extinction periodicity ???

Since the orbital period of the Sun around the center of the Galaxy is about 250 million years then every 100-150 million years the earth is in a spiral arm which is a region of high supernova density. It is possible the earth could be subject to high radition from these supernova.

Deep Sea Thermal Vents:

Many organisms are found living near hydrothermal vents, including tubeworms (see picture, left), mussels, crabs, squat lobsters, limpets, alvinella worms, scaleworms, zoarcid fish, and octopus. Given the absence of sunlight, no plants are found here. Instead, microbes (bacteria and Archaea) form the base of the food chain. These microscopic organisms are chemosynthetic, using energy from hydrogen sulfide in the vent fluid, and oxygen and carbon dioxide from the seawater, to create simple sugars. Some microbes live symbiotically with organisms such as tubeworms and mussels. Others are free-living and may be found growing on many different surfaces, including rocks, tubeworm tubes and other animals, and even inside chimneys.

Impact events at the Earth's surface likely do not disturb the biochemical environment of the deep sea thermal vents. That environment, therefore has been relatively unchanged for billions of years and hence may serve to renew the bio-chemical chain of life independent of the condition of the Earth's atmosphere and its surface. The discovery of these deep sea thermal vents has opened a new area of biological research known as "Life in Extreme Environments" and has strongly raised the possibility that instead of oceans, perhaps all life needs to start is heat.