|I wonder: How were animals and men first exist. I
know that at first they were cells, but how were
they so much that turned them into animals, and
then into humans? I wonder. How many cells would
that take? I wonder.. how much time does it take
to create a living being?|
|Question Date: 2018-10-11|
Life started as molecules that could create
copies of themselves, which then organized
together in structures called cells that could
reproduce copies of themselves, and finally into
multicellular organisms that coordinate
different cell types in order to create new
animals with similar DNA.
The human body has ~40 trillion cells in it,
and those cells have evolved over time to create
tissues with different functions (such as your
stomach and heart tissue) to create one
functioning organism. Unlike the
unicellular organisms we evolved from, human
cells usually can't live on their own because they
have become interdependent on one another.
As you can probably imagine, this process from
single celled organisms to multicelled organisms
took an enormously long time. The first signs
of life around 4 billion years ago!!
The first multicellular organisms only evolved
about 1 billion years ago, and the first humans
appeared about 15 million years ago. However,
those humans would be unrecognizable to you and me
- our species (homo sapiens) only showed up in the
fossil record 1 million years ago!
It's very hard to imagine that amount of time in
your head. If you were to compress all 14 billion
years of history (from the big bang all the way to
present day) into one 12-month calendar, it would
look something like this (Source: Wikipedia):
In one year representing the past 14 billion
years, the first signs of life wouldn't show up
until September. The first multicellular organisms
appear 3 months later in December. However,
homo-sapiens don't show up until December 31st!!
Modern human history fits within a single
compared with the rest of the history of the
Thanks for the great question!
Planet earth formed about 4.5 billion years ago,
and the first life appeared not too long after
that, 3.5 billion years ago at the latest. As you
know, the first lifeforms were relatively simple
single-cell organisms like bacteria. The quick
appearance of life on earth means that it
potentially does not take too much time for life
to take root, should the conditions be right.
About 500 million years after the first
single-cell living things, the first multicellular
organisms appeared. Then over the next few
billions of years, the earth saw the evolution and
extinction of millions of different species of
multicellular life, including plants and animals
like the dinosaurs.
One branch of this web of life
eventually lead to the evolution of modern humans.
Humans are massively multicellular, with recent
estimates suggesting each human has 37 trillion
(37, 000,000,000,000) cells.
You ask, how did life change from a simple single
cell to the complexity of the human body? A
biologist named Charles Darwin was the first to
explain how living things got more complex over
time with the theory of evolution by natural
selection. Evolution occurs because there is
variation among livings things. Those individuals
that happen to be most fit with their environment
are best able to survive and reproduce their
traits. Over time, certain traits die out and
others survive. With the accumulation of new
complex traits, entire new species, including
humans, evolve. Compared to a human lifespan,
evolution is a slow process and so it has taken
millions of years for humans, and other complex
organism, to evolve.
Thanks for the great question!
The processes of humans evolving from animals and
animals evolving from single cells are areas of
active research. Scientists have found, by
analyzing the similarities of genes and proteins
in different organisms, that some members of a
given population will have a trait encoded in
their DNA to survive a given environment better.
For instance, under somewhat harsh conditions, a
few members of a group of single cells may
cooperate with one another better than the
the group. Some time later, something changes in
the environment such that there are now fewer
nutrients, and cooperation among cells makes it
easier for cells to survive and grow. In this
case, the cells that have genes that allow them to
cooperate will survive better than the cells that
do not have these types of genes. Over time, the
initially few cells will take over the group as
the dominant type of cells. Our current theory is
that the genetic traits and environmental
pressures resulted in the evolution of
multi-celled organisms and eventually humans.
I'm not sure we yet understand if there is a
critical number of cells required to go from
single-celled organisms to a multi-celled
organism. I suspect that the number differs with
environmental differences and other factors. In
terms of time, we estimate that it took many
thousands of millions of years to go from the
first single-celled organisms to the first
multi-cellular organism, and then hundreds of
millions of years to get to animals, and finally
many hundreds of millions of years to go from
animals to the ancestors of modern humans.
the concept of living beings, we would need
definitions of "living". Biologists usually see
single cells as living entities, so we could say
that it took a few hundred million years for
living things to appear (first organisms that look
like prokaryotes, or cells without nuclei).
All animals grow from a single cell created
fusion of its father's sperm and its mother's egg.
The DNA within that cell contains all of the
necessary instructions that tell that cell how to
divide and grow into an adult animal, as well as
what type of animal that embryo will become.
Animals' bodies are not created by multiple cells
You contain between fifty and a hundred trillion
cells, all descended from that one cell when your
father's sperm met your mother's egg.
We're still trying to figure out how the earliest
molecules came together in so many complicated
ways to form living cells. Here's my idea about
where that might have happened:
The oldest cells are almost as old as Earth itself
- more than 4 Billion years. Then, it took more
than a billion years before there were cells with
a nucleus, like our cells, and hundreds of
thousands of years more before there were plants
or animals with more than 1 cell! That happened
almost 2 billion years ago. It was only a million
years ago that there were ancient people on Earth
who were even a little like us.
So it all happened very slowly, and we're still
discovering how it all happened.
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