Why is it said we have all evolved from a single cell?

when the earth was first formed, there was no life on it.its surface was boiling hot,and had giant volcanoes that threw fiery rocks,dust,and gases into the air.
The form of life was an organism consisting of only one single cell.these single celled organisms developed into multi cellular organisms that first plants, and then animals,evolved. so we can say that we have all evolved from a single cell!

Which came first,Plants or Animals?

Have you ever wondered what it must have been like millions and millions of years ago,When the earth came into existence?Were there plants and animals then?
It is estimated that the first life forms on earth were primitive,one-celled organism that appeared about 3 billion years ago in the ocean.these single celled organism gradually became multi-cellular organisms.in time,the ocean began to fill up with more complex forms of life.
one of the first forms of life was very simple plant celled algae.Different types of plants came later on.Animal life appeared only after the appearance of plant life.............