We think that the basic chemicals that were present when the earth cooled were sufficient to make the genetic material RNA and life came from that. The first life were single celled orgnanisms (like amoeba and bacteria) as life evolved more complex forms emerged. The first ever animal was probably the trilobite.
This field of study of is called “abiogenesis” – how you can get like from inanimate chemicals or objects. The best theories are the ones that focus around how simple molecules, containing only the elements commonly found 4.5 billion years ago, could come together to self-replicate i.e. to start copying themselves using building blocks found in the environment. The best ideas relate to molecules like DNA, or its close relative RNA, which exist inside all of our cells. Every time our cells divide, the DNA has to be duplicated (copied), and this is done almost entirely by DNA and RNA themselves.
So, can DNA- and RNA-like molecules form spontaneously in the environment? Yes, from mixtures of compounds like ammonium, carbon dioxide, water and sulfuric acid, either hit with electricity (lightning) or in freezing conditions. So, there are lots of hints, but the details are unknown and no-one has actually repeated it in the lab yet.
Ok – I’m a bit shakey on this one as it’s not really my area.
I think that initially they were simple cells floating around in the sea, then these developed into more complex cell based life, which gradually developed into the first animals – sponges seem to be the first animals and they seem to be able to propagate by fragments of them falling off and developing into new sponges. Things gradually got more complex from there until we reached animals that actually give birth
That is a really good question Luke, and one I honestly don’t know the answer to. Actually, I don’t believe anyone does. Although there are many theories, we still don’t fully understand how life first originated on Earth. If perhaps we can understand that, we could create life somewhere else – but would that be a good thing?
There are 2 main theories on this one.
One theory is the evolution of animals and humans from a mix of chemicals via the mechanism of natural selection.
The other is the intelligent design theory which says that natural selection could not have resulted in such complex designs and that therefore an intelligent creator had to be involved in designing them.