I have a thought experiment for you people: to take a ride in a time machine backwards in time. What do you expect to see?
I'll imagine going to around when life originated on Earth, nearly 4 billion years ago.
The "continents" are totally unrecognizable, being much smaller than present-day ones, even though you can recognize some of them as being parts of northern Canada, southern Africa, Australia, and other present-day places with very old continental crust.
The oceans are frozen over; the atmosphere is freezing cold and mostly N2 and CO2 with essentially no free oxygen -- the Sun is somewhat fainter than it is today.
The Earth's land surface looks much like Antarctica, but without very much snow -- the planet is too cold for much water to evaporate. But there are some warm spots -- there are plenty of volcanoes and hot springs and the like, more than today.
There are lots of impact craters; over the last few hundred million years, the Earth had taken a beating from a swarm of asteroids, much like those which created the Moon's "seas" of now-solid lava.
The Moon is significantly closer than it is today, allowing you to see more detail on its surface, and making eclipses more common.
The stars are totally unrecognizable; only a few of the visible present-day ones, like Alpha Centauri, were in existence at this time, and those were elsewhere in the Galaxy. And nearly all those that you see are long gone, having become white dwarfs, neutron stars, or black holes long before the present day. And those still in existence are elsewhere in the Galaxy.
Any life? None apparent on the surface, so you look in hot springs and undersea hydrothermal vents and the like. And not much evidence of life there either. You check on the chemistry -- a big spectrum of organic molecules with various sizes, and a racemic one at that (no preferred molecular handedness).
You look and look, and then you find a hydrothermal vent that shows a curious reduction in variety of organic effluent -- and evidence of handedness! You take some of the mud and analyze it -- that reduction in variety and that handedness is more evident, and you discover that some of the molecules are capable of making copies of themselves! And in the chemical reactions that happen in this mud, you also discover the beginnings of a metabolic network, one that prefers to make only a certain subset of possible organic compounds. A network associated with those self-reproducing molecules.
You've discovered some of the first life on earth!
You look more closely, and you discover that these self-reproducing molecules are polymers that have some sequence features in common with some reconstructed ancestral biomolecules, and you conclude that you could well be looking at a long-ago ancestor.
™ Version 4.2
