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The supermassive black hole at the centre of the Milky Way may have been crucial to the evolution of life in the galaxy.

These days, the black hole, known as Sagittarius A*, is relatively calm. But there are hints that millions of years ago it may have been much more active, swallowing down matter and spewing out high-energy radiation including large amounts of X-rays.

Xian Chen at Peking University in Beijing, China, and his colleagues simulated how these X-rays would affect the abundance of chemicals crucial to the evolution of life: water and organic molecules such as methanol.

When high-energy photons hit molecules, they can knock away electrons. Such molecules then tend to be more likely to latch on to other atoms or molecules, as do those free electrons, so this interaction can kick off a cascade of chemical reactions.

Those reactions can produce larger and larger molecules, eventually resulting in the complex compounds required for life to evolve. 鈥淯ltimately, maybe after billions of years, this little energy from a little photon created close to a supermassive black hole becomes part of a life,鈥� says Chen.

The researchers found that even as far as 26,000 light years from Sagittarius A*, which is about where Earth is, many organic molecules would be present at much higher levels if the black hole was once active than if it never was. This effect could remain for millions of years after the activity ends, they calculated.

鈥淩ight now, the black hole is starved, but it鈥檚 possible that a few million years ago it was shining up to 100 million times brighter,鈥� says Avi Loeb at Harvard University. 鈥淚t could have influenced the material that made the sun, the building blocks of life that we find on Earth.鈥�

It is important to take this sort of temporary event into consideration when thinking about the necessary conditions for life to develop, says Loeb. Because this kind of radiation can also be extremely harmful to planets too close to the black hole, blowing away their atmospheres and sterilising their surfaces, there might be a fine line for planets to walk in terms of where in the galaxy they could host life.

Finding this line is still difficult because we don鈥檛 know exactly what conditions are required for life. 鈥淪o far, we have established the link between the activity of the supermassive black hole and the formation of some basic building blocks of organic and prebiotic molecules,鈥� says Chen. 鈥淏ut from building blocks to a full-fledged life is a long way, and there are still many missing pieces.鈥�

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