Okay, first of all - this question itself makes several assumptions - natural ones to make - but ones that have been questioned.
BOTH COULD BE TRUE - LIFE COULD START MANY TIMES BUT ONLY ONE SURVIVE
- both could be true. It's quite possible that life started on Earth many times before "modern life" finally took off.
For that matter in the early solar system Earth was frequently hit by large impacts. The impact that formed the Moon would surely have sterilized early Earth as it made the entire surface molten. So earlier lifeforms could have started - but gone extinct. Or started, got thrown into space by a giant impact - and returned to Earth later on.
And later impacts could have been pretty devastating also - especially if the very first lifeforms were quite fragile. Maybe to start with they could only survive in particular habitats (say deep sea vent).
Then - you would have had intermediate states - protobionts, things that are almost life but not quite. And then - life that reproduced - but not very exactly - before all the modern error correction got developed. At what point does it count as life, if it almost reproduces, but with many errors - and in tiny "cells" just say 4o nm across or smaller - is that life - or just an unusual chemical reaction? Supposing also it has no dormant state and gets destroyed as soon as the surrounding chemistry changes?
Also life that flourished for a while, went extinct, but later life used its organic remains, maybe including DNA fragments (or precursors of DNA such as PNA or TNA etc), to create new forms of life.
LIFE COULD HAVE STARTED SOMEWHERE ELSE, NOT ON EARTH
So - several options here.
- Life could have started on Mars or Venus - both had global oceans probably, at same time as Earth - and the early solar system was full of many impacting bodies spreading life through the solar system, if it was already there and robust enough to be spread
- Life could have started in "planetary embryos" - Moon sized bodies - but water rich, covered in early seas.
- Life could have started inside comets (smaller objects but still, liquid inside) or inside moons with icy crusts
- Life could have started in another solar system before our one, and been transferred to ours as a result of a star passing through our nebula - and it's planets also - either before the Earth formed, or after, during the bombardment period.
PLACES WHERE ALTERNATIVE FORMS OF LIFE COULD EXIST TO THIS DAY - THAT MIGHT HAVE SHARED EARLY EARTH BEFORE DNA TOOK OVER
- perhaps on Mars,
- or on Ceres or other large asteroids (is a small chance that it has habitats on it).
- Or might find meteorite fragments on the Moon buried deep below the surface that originated on Earth, Mars, or even Venus, from the late heavy bombardment - first few hundred million years of the solar system - quite possibly with organics in them - and also fragments of embryo planets that no longer exist.
(only chance to find out about early Venus most likely as its entire surface gets resurfaced by volcanism every few hundred million years). - Remote chance that Venus still has life in its upper clouds - which might be unrelated to present day Earth life
SHADOW BIOSPHERE - POSSIBILITY OF DIFFERENT FORMS OF LIFE ON EARTH ITSELF
Though all life on Earth as far as we know is based on DNA - that might be because we know how to look for DNA.
For instance - if you search for archaea - one way to test is to look for DNA fragments. Would that find non DNA based life?
We rely on "cultivable" microbes - but what if there are forms of life that we don't know how to cultivate that just aren't interested in the media we put out for them to flourish in?
That's the problem of Microbial Dark Matter
Most microbes can't be cultivated or characterized. Typically only 1% of microbes in a sample (1% by number rather than by species) can be cultivated, rest are far harder to study.
So - if that's the state of play for DNA based life -what if some of those numerous unstudied microbes that we can see in microscopes etc - but can't cultivate - are not using DNA at all? How would we know?
Then - there's also the possibility of life that is smaller than any life that we know. The smallest possible modern living cell is calculated to be about 200 nm - and as it happens we can observe cells that small. And coincidentally - that's about the same as the optical resolution of the best optical microscopes.
But - the earliest lifeforms on Earth must have been far smaller, estimates make them perhaps 40 nm in size, and can't have been nearly as complex as present day life with its million different chemicals in a complex dance making up the cell.
So - what if simpler cells like that still exist to this day? Would we spot them? They might just seem like biological biproducts of the larger cells.
Well we do spot strange microbe like structures at around the right scale, called nanobes.
Microscopy-UK full menu of microscopy and microscopes on the web
Most think they are some kind of inorganic process happening at the nanoscale level. But - we can't see them in real time, only frozen in time as prepared specimens under electron microscopes - as they are too small to see with optical microscopes.
What if these really are lifeforms? What if they are growing and evolving and reproducing?
Then they couldn't be modern life as they are too small for all the machinery of DNA. And couldn't have derived from the Last universal ancestor as that would be too large.
They could be from the same Ur-organism - i.e. having a common origin - but much earlier than the branching off point for most modern life.
Or could have a separate origin altogether.
Either way - they could be an example of a Shadow biosphere
Other things could similarly be signs or them - such as for instance the mysterious "rock varnish" - not well understood - but - the shadow biosphere could be everywhere - just not recognized, if it can't be cultivated using normal cultivation methods and can't be amplified by the methods used to find DNA fragments.
As we develop better tools for investigating these tiny sub optical resolution structures, hopefully we can get some answers to this.
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
4.8m answer views110.4k this month
Top Writer2017, 2016, and 2015
Published WriterHuffPost, Slate, and 4 moreHuffPost, Slate, Forbes, Newsweek, Business Insider, and Science 2.0