The very earliest photo-synthesizing plants on Earth, about 3
billion years ago, had only one cell, and they reproduced the
way most one-celled creatures do, by mitosis: splitting
themselves in half. By about two billion years ago, some of
these cells began to split themselves in a more sophisticated
way, called meiosis. When cells use meiosis to reproduce,
there's a daddy cell and a mommy cell. Each parent cell makes a
new cell with a copy of half of its DNA, and then the daddy's
half cell mixes with the mommy's half cell to make a new baby
cell. That's how you were made, and that's how baby plants are
made too. A cell that reproduces using meiosis has two possible
forms: the kind with only half the DNA, or haploid cells, and
the kind with all the DNA, or diploid cells. Some cells spent
most of their time as haploid cells, only becoming diploid for a
short time just to mix the DNA up. Other cells spent most of
their time as diploid cells, only becoming haploid for a short
time to mix the DNA up. (You spend m