Despite the countless scientific and technological advancements over the course of human history, there are still a few big questions we’re no closer to answering now than we were in the stone age. One of the biggest — or perhaps the biggest — is whether or not we’re alone in the universe.

We know that space is filled with galaxies, and those galaxies are packed with stars, and those stars are orbited by a seemingly endless number of planets. We think that at least a chunk of those worlds are a lot like Earth, but whether any of them are home to intelligent life is anyone’s guess. In a new article on LiveScience, Nick Longrich, a senior lecturer at the University of Bath, suggests that the odds are slim.

There are a number of hurdles standing between any given planet and the emergence of intelligent life. The planet has to be rocky, with suitable temperature and a robust atmosphere. Based on what we think we know about how life got its start here on Earth, multiple factors have to line up perfectly, with liquid water on the surface, and the right mix of molecules has to be present.

But even if all of that happens, and microbial life takes root, evolution will ultimately determine what life forms stand a chance. Using Earth as an example, it took billions of years for microscopic life to evolve into simple organisms and hundreds of millions of additional years for more complex life to emerge.

Even then, our planet was packed with complex organisms for hundreds of millions of years with no “intelligent” life to show for it. Dinosaurs and other creatures dominated the land and sea, with evolution deciding who lives and who dies, but no intelligent life emerged.

Then something happened. In the wake of an asteroid impact that killed off the majority of life on the planet, primates got a new lease on life, and over a relatively short period of time, their brains began to grow in both size and complexity. Fast forward another several million years and here we are, with civilization and modern humans only inhabiting the Earth for a tiny fraction of our planet’s history.

As Longrich argues, there are many, many places along Earth’s timeline where evolution could have stalled out and prevented our existence entirely:

Imagine that intelligence depends on a chain of seven unlikely innovations — the origin of life, photosynthesis, complex cells, sex, complex animals, skeletons and intelligence itself — each with a 10% chance of evolving. The odds of evolving intelligence become one in 10 million.

Those percentages are just basic guesses since we really have no idea how likely or unlikely any of those individual events are to occur on a planet other than Earth. Longrich notes that if the percentages are even lower, in the 1% range, the odds of intelligent life emerging on a habitable world are around 1 in 100 trillion.

Combine that with the size of the universe and the odds that a habitable world with intelligent life is in our neck of the cosmic woods and you start to understand why the odds of running into an intelligent alien civilization are so impossibly tiny.