estion of whether life on Earth is unique in the Universe is as old as mankind itself. Space agencies such as NASA are currently training its telescopes such as TESS (Transiting Exoplanet Survey Satellite) in an attempt to answer the ultimate question by searching for habitable exoplanets. But a scientist has now revealed how the futility of searching for alien life, after studying the "fluke" of life on Earth. Dr Nick Longrich, a paleontology and evolutionary biology expert at the University of Bath describes the occurrence of intelligent life as an "improbable event occuring rarely – or once." He wrote in The Conversation: "Our evolutionary history shows that many key adaptations – not just intelligence, but complex animals, complex cells, photosynthesis, and life itself – were unique, one-off events, and therefore highly improbable. "Our evolution may have been like winning the lottery … only far less likely." Humans are incapable of conceiving the size of the Universe. The Milky Way has more than 100 billion stars, and there are at least a trillion galaxies in the visible universe, the fraction of the perceivable Universe. The famed Fermi paradox suggests alien life should have been encountered by now. Even if habitable worlds are rare, the sheer number of planets suggests lots of life should exist. The universe is large, and old, with time and room for intelligence to evolve, but there’s no evidence of it. Dr Longrich offers a different approach to the ultimate question by studying 4.5 billion years of Earth’s history, examining where evolution repeated itself or not. He said: "Evolution sometimes repeats, with different species independently converging on similar outcomes. "If evolution frequently repeats itself, then our evolution might be probable, even inevitable." He lists several examples of convergent such as the striking resemblance between the thylacine marsupial and the wolf, and dolphins and extinct ichthyosaurs, both of whom evolved from different lineage. However, Dr Longrich then reveals "the catch" - complex animals evolved once in life’s history, suggesting they are "improbable". He said: "Surprisingly, many critical events in our evolutionary history are unique and, probably, improbable. "Sex evolved just once. Photosynthesis, which increased the energy available to life and produced oxygen, is a one-off. For that matter, so is human-level intelligence. "There are places where evolution repeats, and places where it doesn’t. "If we only look for convergence, it creates confirmation bias. "Convergence seems to be the rule, and our evolution looks probable. "But when you look for non-convergence, it’s everywhere, and critical, complex adaptations seem to be the least repeatable, and therefore improbable." The University of Bath lecturer proceeds to describe how one-off innovations, critical flukes, may create a "chain of evolutionary bottlenecks or filters." He added: "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 percent chance of evolving. "The odds of evolving intelligence become one in 10 million. "But complex adaptations might be even less likely. "Photosynthesis required a series of adaptations in proteins, pigments and membranes. Eumetazoan animals required multiple anatomical innovations (nerves, muscles, mouths and so on). "So maybe each of these seven key innovations evolve just 1 percent of the time. "If so, intelligence will evolve on just 1 in 100 trillion habitable worlds. "If habitable worlds are rare, then we might be the only intelligent life in the galaxy, or even the visible Universe."