Well, after the creation of the Universe, it’s hard to say, because life can possibly have developed at many places at different times. But on earth, life started out as unicellular organism (that means made of only one cell) of a very simple kind, about 3.6 billion years ago. We have evidences of this from fossils that are dated to be 3.5 billion years ago!!
These very simple cells are called prokaryotes. What makes them simpler than, say, the cells in our body is that 1) their membrane is much simpler, it’s made of a single layer of lipides (that’s just fat, like if you drop oil in water, it’ll make little droplets. the outer layer of those cells is exactly like the outer layer of those droplets of oil), 2) they have no nucleus, which means their genetic material was free to float around unprotected throughout the cell (that also mean that they were MUCH more prone to mutations!!) , and 3) they don’t have mitochondria and chloroplasts. Mitochondria are the organelle (like an organ, but for a cell) that transforms sugar into energy in animal cells, and chloroplasts are the organelles that do photosynthesis. Procaryotes get their energy through much simpler processes that happen through the cell membrane.
Those first living organisms are believed to have evolved from ‘protocells’, called micelles, which really are just tiny droplets of oil (technically the oil molecules are called phospholipids). Those micelles provided a boundary that could separate the life processes from the outside world. There has been experiments (one of which was awarded a Nobel prize in 2009) that showed that just those simple micelles can do things like store energy and exhibit primitive forms of Darwinian selection. I find this absolutely mind-blowing!
As a side note about life outside Earth: few months ago I read a very interesting paper from a physicist at Harvard. That paper was suggesting that, much earlier in the history of the Universe, around 10 to 17 million years after the Big Bang there was an epoch when the temperature of the Universe was so much warmer than now that water could have been in liquid form on any rocky planet or asteroid, regardless of their distance from a star. That is because the radiation leftover from the Big Bang, which we call cosmic microwave background (CMB for short) at that time was much warmer than today, it was actually between 0 to 100C, which is perfect for liquid water! In contrast, today, the CMB has a temperature of 2.7K which is -270C, so space is really cold!!
These early rocks basically had a constant warm shower of light raining on them. Because of this they could have potentially hosted water chemistry and seen the formation of the very first living organism. This is a bit crazy, but I find this hypothesis really fascinating!! 😀
Hi u12kopaczh! This is quite a tough question. The Universe was created 13.8 billion years ago. If we assume that the only place where life can form is the Earth, then that means the earliest we could have had life would have been 4.5 billion years ago when the Earth formed. However, the initial formation of the Earth probably wasn’t very suitable for life to form due to it being too hot, so we probably would have had to wait for a long time before the Earth cooled and the liquid water environment became available. Current estimates suggest that the first single celled organisms (prokaryotes) were on Earth about 4 billion years ago.
However, what if there is life on other planets/moons in the Universe? Maybe the conditions for life developed earlier on those planets and some completely alien bacteria formed there first?