First life form on earth

Does anybody  know the first life form on earth, was it a plant or an animal
in Charles Darwin 's origin of Species? I am inclined to think it is a plant
and  I read from somewhere that it was   green algae.   But  I also came
across somewhere that it was  sponges.   That leads to the next question
namely,  how did the plant become an animal?
Thanks
Khoon.