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When did you begin?

YOU almost undoubtedly know the date, possibly even the hour, you were born. Whether you are past celebrating rather depends. But reflect on the big picture, and the truth about when you began is too epic, and possibly a little too confusing, to be captured by a terse entry on a birth certificate.

That story begins in the deep cosmos. As anyone with a passing interest in Joni Mitchell鈥檚 back catalogue knows, we are stardust. It鈥檚 a nice line, and it also happens to be true, says , an astrophysicist at the Lockheed Martin Advanced Technology Center in California.

Most of your body鈥檚 trillions of atoms, from calcium in your bones and carbon in your genes to iron in your blood, were forged by nuclear reactions in ancient stars, either when they were burning or when they ended in fiery supernova explosions. Those atoms were recycled through the births and deaths of more stars until, at some point, they escaped for a while. 鈥淥ur solar system captured these elements to make Earth and everything on it,鈥� says Schrijver 鈥� including you.

In that sense, we can鈥檛 know exactly when we began: it depends how many generations of stars our atoms cycled through. But each of us is at least 4.6 billion years old, the age of the solar system, and perhaps as ancient as the universe鈥檚 first stars, which appeared some 13.7 billion years ago, just 100 million years after the big bang. The hydrogen within you was probably forged in the big bang itself.

So much for the physical, atomistic you. But what about you as a living, breathing biological organism? Here your timescale shortens, but the uncertainties hardly disappear. 鈥淥ne thing I can say with absolute certainty is that there is no scientific consensus as to when independent human life begins,鈥� says , a developmental biologist at Swarthmore College in Pennsylvania.

For many centuries, a life began with the 鈥渜uickening鈥�: the first time a mother felt her baby鈥檚 kick. These days, those expecting a child can hear a fetal heartbeat much earlier, and can even see the blurry outline of a face, thanks to ultrasound. That makes defining a starting point more complicated (see 鈥�Four points when you might have begun鈥�).

Some aren鈥檛 convinced that is even a scientific discussion. 鈥淭o many biologists, the onset of personhood is an issue for emotions and politics, not science,鈥� says Gilbert. Ultimately, it all comes down to how you define 鈥測ou鈥�: as a collection of atoms, an agglomeration of cells 鈥� or something more.

Our physical senses develop gradually in the uterus and after birth, but you remain completely ignorant of your you-ness until you develop a sense of self. A psychologist might say that you only really become you once you are able to reflect on your own consciousness from the perspective of another person.

The development of this 鈥渢heory of mind鈥� tends to happen as we approach our second birthday. We begin to equate our image in a mirror with ourselves, and to use self-referential language, such as 鈥淚鈥�, 鈥渕e鈥� and the classic 鈥渕ine鈥�. By the time we are 3, most of us have added self-referential emotions such as embarrassment, pride, guilt and shame. Soon after, we begin to store the autobiographical memories that underpin a stable, continuous sense of self.

But that continuity might well be an illusion (see 鈥淎re you always the same person?鈥�). If so, another answer to the question 鈥渨hen did you begin?鈥� might arguably be that you have no beginning, just a now.