The universe is thought to be 13.7 billion years old. Matter and energy can not be created or destroyed - they can only change forms or convert from one to another.
By implication, that means you are 13.7 bilion years old. Or more precisely, the matter that comprises your body has existed, in some form or other, for 13.7 billion years old. This matter is constantly being recyled and replaced with new matter, matter that is also 13.7 billion years old. What your age represents is the length of time that the atomic pattern that is the blueprint of your being has been in the world.
At the start of the universe the only elements that existed were hydrogen and helium. All the other elements, including the organic elements that are the building blocks of life on Earth, were produced inside of stars. This means that at some point, the carbon and nitrogen and oxygen that is the bulk of your body (excluding hydrogen) was formed billions of years ago inside of a star by the process of nucleosynthesis. As Carl Sagan liked to muse, we are quite literally made of star dust.
The Earth formed about 4.5 billion years ago. One billion years later, evidence of life first emerged. There currently is no accepted theory as to how life began, but I suspect time will bare out the finding that life began with some sort of autocatalytic process where proteins that filled the oceans began to self-replicate. If this turns out to be correct, this means we are the Earth come alive.
Speculation aside, you represent an unbroken chain of life stretching back to the time when life first appeared. It took 3.5 billion years of evolution to arive at you. You represent genes that have traveled billions of years, changing and evolving along the way, to arrive in combination at the production of you.
All life on Earth shares as a common ancestor that first life form that rose in the sea. That means you are related to every living thing on the planet. It is a favorite habit of Creationists to hang their argument of personal incredulity upon the notion that we evolved from monkeys (we didn't, we share a common simian ancestor with them) but I've never understood this, because if you're going to make such an argument (which is a logical error, anyways) one can find much more outrageous claims from evolutionary theory. Yet these claims are true.
The common ancestor of all multi-celled animals was probably something resembling a sea sponge. Yes, one of your great........great grandparents may have been a sponge. Strange but true.
Approximately 440 million years ago one of your grandparents was swimming in the ocean as a fish. Thankfully for you, he managed to not get eaten before reproducing and passing on the genes that would eventually become you along their billion years journey.
One hundred millions later - 340 million years ago - those genes had evolved and brought your ancestor out of the sea. This ancestor was some sort of amphibian.
Another hundred million years marked a tough time for your ancestor, as at the end of the Permian there was a mass extinction in which 90 - 95 % of life on Earth was wiped out. This grandparent was now a mammal-like reptile, probably something close to a therapsid. If he hadn't managed to survive the greatest extinction in Earth's history, you would not be here today.
More time passes. Its the age of dinosaurs. Your ancestor is now a small, shrew-like mammal. And around 65 million years ago, he manages to survive a meteorite strike that was so cataclysmic that 85% of all life went extinct. According to Richard Dawkins, the sound of the collision would have been so thundering that most life on the planet would have been struck deaf. Your possibly deaf, possibly blind, possibly burned ancestor dodged a proverbial bullet, fortunately.
This great .... great grandparent of yours survived and its descendents proceeded to evolve, slowly and slightly, so slowly and slightly that our minds, which have a habit of trying to view the world in discrete categories, have a difficult time grasping how such changes could ever occur. But occur they did, and around 40 million years ago - 25 million years after the great escape of your other ancestor - your ancestor was now swinging from a tree. He was something similar to a monkey.
It took another 40 million years of evolution to get from him to you. That's 40 million more years of your direct predecessors managing to win the game of life and successfully pass on their genetic material to the next generation.
If you think this is remotely interesting, then check out The Ancestor's Tale by Richard Dawkins. Its written as a backwards pilgrimage through time, with humans being joined along the way at rendevous points in history by other living organisms where they share a common ancestor (concestor) with us.
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