It’s a common tenet of evolutionary biology1 that all earthly life (plants, animals, bacteria and in fact, all living things) evolved from a single-celled organism that lived several billion years ago.2 (Links to the end notes are not working…please refer to them at the end of the article.) Not from a type of organism3, but from one single individual cell.
The image here is of a tardigrade, an ancient tiny primitive animal that still lives today. The tardigrade is not our ancestor, but merely a form of life with which humans – and all other forms of life on Earth – shared a common ancestor going back more than 3 billion years.
Although it is impossible to know the exact timing, the earliest generally accepted evidence for life dates to about 3.7 – 3.8 billion years ago. It is likely that suitable conditions had been present for some time before that. In fact, some have suggested that suitable conditions may have existed as far back as 4.2 or 4.3 billion years ago. The Earth itself is thought to have formed about 4.5 billion years ago, but initially being molten, conditions were too hot for several hundred million years for any chance of life.
There is a chance that forms of life may have arisen or have been “seeded” (Panspermia4) on Earth by organisms carried here on asteroids or comets that collided with the young planet many times in the early days, but were wiped out by the harsh conditions before they had a chance to spread. Certainly one instance of life, whether formed on Earth originally or brought here through collision with some ancient space rock, survived, and we are its descendants. At some point not long (on a geologic scale) after the first life appeared, the environmental conditions changed so that the “formation” of life could no longer happen. This obviously does not mean that life was impossible, but that life on Earth dates from that time, since conditions (e.g., temperature, atmospheric composition and so on) no longer allowed the “creation” of “new” life. This continues until today, as evidenced by the fact that no new forms of life are seen to come into existence. (Even if new life were to form, it would be extremely primitive and likely would be quickly wiped out by the oxidizing character of our current atmosphere.)
You could argue that numerous instances of the formation of life occurred and have evolved into the various lifeforms we see today. After all, humans and horses are very different from begonias and bacteria. However, careful analysis of form and function of living species shows evidence of a common ancestor, leading to Darwin’s version of evolution, and detailed DNA analysis strongly indicates that all forms of life on Earth today are evolved from one single form. Likely one single cell5.
A bit of a diversion here: all of the basic building blocks of matter, including the protons in every atom in your body, formed within a few minutes after the Big Bang, nearly 14 billion years ago6. Some have undergone transformations, of course, such as most “heavy” elements (e.g. Carbon, Oxygen, Calcium and so on) that were later produced in the cores and explosions of stars billions of years ago (but perhaps billions of years after the Big Bang). Still, all of the most fundamental particles in all atoms that exist today formed more than 14 billion years ago. Although you as an individual may have come into existence only a few decades ago, the material of which you are composed is as old as the Universe.
Now back to my original heading: there is ample evidence — some would even say overwhelming evidence — that all life on Earth today arose from a single ancestor more than three billion years ago. If that is not interesting enough, it is my contention that we all share in the same “spark7” of life that arose those billions of years ago.
Your life did not begin when you were born (although of course you can argue that you as an individual, did). There was no miraculous creation of life at your birth, nor even when you were conceived. It came to you as a continuation of your parents’ lives, who got it from their parents, and on and on backwards through time. Although most if not all of your direct ancestors have died, it was not before they passed their life on. It is not possible to trace in practice, but your life is a continuation of that first spark, carried down in continuous8 fashion through millions of generations from single cells to more complex organisms, to the first animals, mammals, primates, humans… to you.
The takeaway is that the life that dwells within you now actually started billions of years ago.
When you die, your personal branch on the “tree of life” ends. That one specific continuous thread that started with a one-celled organism billions of years ago and came to you through millions of generations of human and pre-human ancestors, something like an Olympic torch, ends. But if you have children, they have received a bit of that same flame from you and can continue it into the future in an unbroken connection to the distant past. The same is true for the roughly 5 quadrillion quadrillion (very rough estimate) other living organisms on the planet today, from blue whales to bacteria.10
Now if that is not something to make you stop and think about your existence, I don’t know what is.
NOTES
1 For the record, I have been writing about popular science for roughly 50 years, but I am not an evolutionary biologist nor a biologist of any kind. My training is in physics. Comments or corrections by anyone more knowledgeable than me are encouraged and welcomed.
2https://bit.ly/40czTZu
3https://en.wikipedia.org/wiki/Evolution_of_cells
4https://bit.ly/3TDyDfw
5https://on.natgeo.com/3niQoVi
6https://bit.ly/3Z6ryVT
7 I use the term “spark” for lack of a better word, and do not in any way mean to imply that I think life is of preternatural or supernatural origin. I simply don’t have a better word to describe something that we do not understand. We don’t know exactly how life arose, and as such I must remain agnostic until a satisfactory explanation is available.
8 It is continuous simply because if it had been broken at any point, you would not be here. In our time, and under the conditions that have existed on Earth for several billion years, life cannot arise from non-living materials, at least as far as exhaustive lab experience has shown. I do not mean to imply that life cannot be “created” in a lab, but simply that the conditions under which it was originally formed have not yet been discovered and apparently do not exist naturally anywhere on the planet at this time.
9 Most of the stars you can see are a few dozen to a few hundred light years distant (still a very, very long way away!) For technical reasons I won’t discuss, without optical aid, human eyes can see stars no more than a few hundred light years. The farthest easily visible star, Deneb, is at least 1,500 light years away, although the exact figure is a matter of debate. The farthest thing we can see fairly easily with just the normal human eye under good conditions is actually a galaxy, the Andromeda Galaxy, about 2.5 million light years away.
10 This refers to the total number of individual organisms at this time, not to the number of species. In addition, this number is pure speculation, based only on various figures online, which vary a lot but are all extremely large numbers from roughly 1025 to 1033. The number I have used is 5 X 1030
Starman's Meanderings 
Links to the endnotes seem not to be working, but you can find them at the end of the text.