We know Earth is special. So are we alone in the galaxy?

The Milky Way is extremely unlikely to contain another technological civilisation

The question of whether human beings are the only intelligent life in our galaxy continues to fascinate. The latest attempt to answer it is made by the eminent science writer John Gribbin, in Scientific American. Gribbin argues that we are almost certainly the only technological civilisation in the Milky Way, because the emergence of intelligence such as ours, dependent on a long sequence of improbable events, is extremely unlikely to have happened more than once.

There are 92 natural elements. The two lightest elements, hydrogen and helium, were made in the big bang about 13 billion years ago. All the heavier elements, the metals, were later sequentially bred in stars. The earliest stars formed when hydrogen and helium gases condensed under gravity until nuclear fusion of hydrogen into helium was initiated.

Stars die and scatter their contents into space when their hydrogen fuel becomes depleted. As time passed after the big bang, the metals forged in stars and scattered into space condensed to form planets. Our solar system was born 4.5 billion years ago. Our sun is composed of 71 per cent hydrogen, 27 per cent helium and 2 per cent metals, the gas mixture from which planet Earth also arose. If our solar system had formed earlier it would probably harbour too few metals to form a rocky Earth.

Our solar system resides in the Milky Way galaxy, a disc of stars about 100,000 light years in diameter. Earth is located in the “Goldilocks zone”, 23,000 to 30,000 light years from the galaxy centre, where conditions allow liquid water, and therefore life, to exist. Closer to the galaxy centre inhospitable events such as supernovae and gamma ray bursts lethal to life are common. Less than 5 per cent of Milky Way stars are in the Goldilocks zone.

Special planet

Earth is a very special planet. It has a thin crust broken into large fragments that slide about on a molten underlay – plate tectonics. Valuable materials from the molten interior well up to the surface, eg nutrients depleted by surface biological life and minerals vital to our technological civilisation. This process also recycles carbon, stabilising Earth’s temperature over the long term.

Earth also has its own moon, probably formed when a Mars-size object collided with the young Earth causing both to melt. Heavy metals from both objects settled into the Earth’s core and much of Earth’s lighter materials were ejected outwards to become the moon. Earth’s spinning metallic core produces a strong magnetic field that shields Earth’s surface from incoming cosmic radiation that would otherwise kill life on the surface of the planet. The gravitational pull of the moon also prevents Earth from wobbling on its axis as it goes around the sun, providing a stable home for life. If the improbable events that produced the moon hadn’t happened Earth would neither have a magnetic field, plate tectonics nor orbital stability and humans would probably never have arisen.

Life began on Earth early, about 3.4 billion years ago, as simple prokaryotic unicellular bacterial life – simple membrane bound sacs containing genetic material (RNA/DNA) plus the minimum life-sustaining chemical machinery. Prokaryotic cells are much smaller than the eukaryotic cells that make up animals and plants.


There was little further development of life for the next 2 billion years, when the first eukaryotic cells were born, obviously a highly improbable development as indicated by the timeframe. Eukaryotic cells apparently arose when one prokaryotic cell ate another but did not digest it. The engulfed cell remained intact and started to do useful things to sustain the new enlarged cell.

Many new eukaryotic life forms arose during the Cambrian explosion 541 million years ago when the basic body plans for most animal life were established. Eventually the first human beings emerged and, after many vicissitudes, evolved to our present state. Humans nearly went extinct on several occasions – eg about 150,000 years ago the human population was reduced to perhaps only a few hundred breeding pairs.

In view of the alacrity with which life got started on Earth, Gribbin concludes that simple prokaryotic life is almost certainly present elsewhere in the Milky Way. But, in view of the improbable sequence of necessary preceding events, a technological civilisation almost certainly is not.

William Reville is an emeritus professor of biochemistry at UCC