In 1650, an Irish archbishop sat down with a Bible and did some math. What he came up with would shape — and constrain — scientific thinking for the next two centuries.

His name was James Ussher. And he calculated that God created the world on the evening before October 23, 4004 BC.

Ussher was no fool. Born in Dublin in 1581, he entered Trinity College at age 13. By 26 he was a professor of divinity. By 43 he was Archbishop of Armagh, the highest Protestant church office in Ireland. His magnum opus, the Annals of the Old Testament, contained over 12,000 references from secular sources and 2,000 from scripture. For the 17th century, it was a serious scholarly undertaking.

The method was straightforward. The Bible’s “begats” — the long genealogical chains connecting Adam to Abraham and beyond — provided a generational timeline. Ussher cross-referenced these with ancient Egyptian, Persian, and Hebrew records to anchor biblical events to known historical dates. He worked backward from the birth of Christ, chaining generation to generation, and landed on a number: the Earth was roughly 6,000 years old.

He wasn’t alone. Cambridge scholar John Lightfoot independently calculated the creation of Adam to precisely 9am on October 23, 4004 BC. The great reformer Martin Luther had favored 4000 BC as a round number. Astronomer Johannes Kepler settled on 3992 BC. In Shakespeare’s As You Like It, written around 1599, a character casually mentions that “the poor world is almost six thousand years old.”

This was the intellectual consensus of the era.

But it was what happened next that made Ussher’s date so consequential. Editors of the King James Bible began printing his chronology in the margins of the Old Testament — right alongside scripture itself. “In the beginning…” became annotated with “4004 BC.” Over generations, the marginal note stopped looking like a calculation and started looking like revelation. Ussher’s number absorbed the authority of the text it accompanied.

By the 18th century, that authority was under pressure.

James Hutton, a Scottish geologist, spent years studying rock formations and reached a radical conclusion. The Earth was not thousands of years old. It was unimaginably ancient. At Siccar Point on the Scottish coast, he observed two layers of rock folded against each other at impossible angles — the product of processes that would have taken millions, perhaps hundreds of millions, of years. In 1788 he published his Theory of the Earth, introducing what would become known as “deep time.”

The reception was not gentle. Hutton’s work was attacked on religious grounds. Critics accused him of promoting atheism. He died in 1797 without seeing his ideas accepted. It took his friend John Playfair’s 1802 summary of his work, and then Charles Lyell’s landmark three-volume Principles of Geology published between 1830 and 1833, to push the scientific community toward accepting an ancient Earth.

Even then, the transition was slow. Some Christian geologists tried to reconcile geology with the Bible by arguing that the “days” of Genesis were actually geological epochs of indefinite length. Others proposed that the rock record was evidence of Noah’s Flood. The debate dragged on across pulpits, lecture halls, and scientific journals well into the 19th century.

Hutton’s concept of deep time became the foundation that made Charles Darwin’s theory of evolution possible. Darwin brought a copy of Lyell’s Principles aboard the Beagle in 1832. Without vast stretches of geological time, natural selection had no room to work.

Today, using radioactive dating techniques, we know the Earth is approximately 4.6 billion years old — a number so far beyond Ussher’s 6,000 years that it barely fits in the same sentence.

Ussher’s date was eventually removed from the margins of most Bible editions. But it took until the early 20th century for the last of them to disappear.

The real story here isn’t about a villain. Ussher was doing the best science available to him with the tools he had. The problem was that his conclusion got printed in the Bible — and once something gets that kind of authority, it doesn’t let go easily. That’s not a 17th-century problem. It’s a very human one.