It happened over 300 years in Holland

Cells
(1) Except for germs like viruses and bacteria, just about every other
living thing on Earth is made of cells. This is probably why cells are
called the building blocks of life.
(2) Most organisms contain many cells. In the ocean, however, there
are some types of creatures that have a single cell. They are called
plankton and are eaten by larger sea creatures such as mussels, fish
and even some species of whales.
(3) Most forms of life are made of many cells. It is estimated that the
average human adult body contains about 10 trillion cells. Placed
end to end, they would stretch around the earth 47 times. A ten year
old has about half that many cells. If you could count them at a rate of
one cell per second, it would take you over 2600 years to complete the
task.
(4) Numbering in the trillions, it is not surprising that most cells of the
human body are very small. In fact, an individual cell is invisible
unless viewed through a microscope. The largest human cell, an egg
cell produced by females when they are able to have babies, is are
about the diameter of a human hair. The smallest, a sperm cell
produced by the male, is one-tenth of the diameter (the distance
across) of a human hair.
(5) Even though they are
quite small, not all cells
are alike. They differ in
size, shape and function
(how they work). As the
picture on this page
shows, bone cells differ
from blood cells and
nerve cells differ from
muscle cells. Each one is
designed to do a
different job. Red blood
cells carry oxygen
throughout the body.
Nerve cells carry
electrical signals to and from our brains to muscles all over our
bodies. Bone cells, which are very rigid, form the skeleton that gives
our bodies shape. Muscle cells contract to move these bones to help
us get around. Stomach cells secrete an acid to digest our food.
Special cells in our intestines absorb nutrients from the food we eat.
And no matter what they do, many of these cells change food into
energy to keep our bodies working.
(6) Cells are packed tightly together. They combine to form tissues,
like skin and muscle. Tissues combine to form organs. Muscle cells
combine to form muscle tissues. Muscle tissues combine to form
organs like your heart. Organs cooperate (work together) to keep us
alive.
(7) All human body cells are covered by a membrane. This layer
holds the contents of the cell in just like the sides of a container. The
membrane lets good things, like nutrients, into the cell. It also keeps
bad things, like germs and bacteria, out.

Amazing Facts

About
Our Bodies
Cells
In four square centimetres of skin there are 3 metres of nerve fibers, 1300 nerve cells, 100
sweat glands, 3 million cells, and 3 metres of
blood vessels.
Except for your brain cells, 50,000,000 of the
cells in your body will have died and been
replaced with others, all while you have been
reading this sentence.
The central nervous system is connected to every