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):

cosmic calendar

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 minute compared with the rest of the history of the universe.

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 rest of 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.

As to 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 by the 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 coming together.

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:

HHansma Mica

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.