Question: how did the first cells evolve into so many different species?

Stuart Higgins answered on 7 Mar 2019:

Cells can experience mutations – a change in their code of their DNA – which can be caused by lots of things in the environment. The DNA is the code that contains all the information about stuff like what chemicals the cell can make, and how the cell works.

Sometimes those mutations are bad, but other mutations might mean that the cell can do something useful it couldn’t before. That useful thing might mean that those cells survive better than other cells without the mutation. So living things containing those cells are more likely to survive and they slowly become a different species. As lots of mutations happen over time, more and more species form, as each species becomes more and more diverse.

Because we come from a common route, living things share different amounts of the DNA depending on how long ago the mutation and split happened. For example humans share about 1% of their DNA with bananas!

This question is what evolutionary biologists study, if you find this interesting, biology would be a good science for you.

https://www.thenakedscientists.com/articles/interviews/mythconception-we-share-half-our-dna-bananas