Based on current thinking, it seems far more probable that life in the universe will mostly be very small and entirely lacking consciousness, in other words, microbial. After all, life on Earth arose pretty much as soon as the environment was stable enough, around 3.7 billion years ago or even earlier. In contrast, lifeforms large enough to be visible without a microscope evolved around 1 billion or so years ago (for photosynthetic plants) and only about 580 million years ago for complex marine animals.
The recent publicity surrounding the seasonal variations in methane on Mars has provided ever more tantalising hints that microbial life may survive in ultraviolet-free shelters near the Martian surface, although it will be some years before a robot mission sophisticated enough to visit sink holes or canyon walls can investigate likely habitats. (As for the oft-talked about but yet to be planned crewed mission, see this post from 2015.)
Therefore it seems that it is worth concentrating on finding biological or pre-biological compounds in extra-terrestrial objects as much as listening for radio signals. The search can be via remote sensing (e.g. of molecular clouds, comets and asteroids) as well as investigating meteorites - bearing in mind that the Earth receives up to one million kilogrammes of material per day, although less than one percent is large enough to be identified as such.
The problem is that this area of research has at times had a fairly poor reputation due to the occasional premature claim of success. Stories then become widespread via non-specialist media in such a way that the resulting hype frequently bears little relation to the initial scientific report. In addition, if further evidence reverses that conclusion, the public's lack of understanding of the error-correcting methods of science leads to disillusion at best and apathy at worst.
One key hypothesis that has received more than its fair share of negative publicity has been that of panspermia, which suggests not just the chemicals of biology but life itself has been brought to Earth by cosmic impactors. The best known advocates are Fred Hoyle and Chandra Wickramasinghe, but their outspoken championing of an hypothesis severely lacking in evidence has done little to promote the idea. For while it is feasible - especially with the ongoing discovery of extremophiles everywhere from deep ocean vents to the coolant ponds of nuclear reactors - to envisage microbial life reaching Earth from cometary or asteroid material, the notion that these extra-terrestrials have been responsible for various epidemics seems to be a step too far.
It's long been known that comets contain vast amounts of water; indeed, simulations suggest that until the Late Heavy Bombardment around four billion years ago there may have been far less water on Earth than subsequently. Considering the volumes of water ice now being discovered on Mars and the Moon, the probability of life-sustaining environments off the Earth has gained a respectable boost.
It isn't just water, either: organic compounds that are precursors to biological material have been found in vast quantities in interstellar space; and now they are being found in the inner solar system too. That's not to say that this research has been without controversy as well. Since the early 1960s, Bartholomew Nagy has stirred debate by announcing the discovery of sophisticated pre-biological material in impactors such as the Orgueil meteorite. Examination by other teams has found that contamination has skewed the results, implying that Nagy's conclusions were based on inadequate research. Although more recent investigation of meteorites and spectrophotometry of carbonaceous chondrite asteroids have supplied genuine positives, the earlier mistakes have sullied the field.
Luckily, thorough examination of the Australian Murchison meteorite has promoted the discipline again, with numerous amino acids being confirmed as of non-terrestrial origin. The RNA nucleobase uracil has also been found in the Murchison meteorite, with ultraviolet radiation in the outer space vacuum being deemed responsible for the construction of these complex compounds.
Not that there haven't been other examples of premature results leading to unwarranted hype. Perhaps the best known example of this was the 1996 announcement of minute bacteria-like forms in the Martian ALH84001 meteorite. The international headlines soon waned when a potential non-biological origin was found.
In addition to examination of these objects, experiments are increasingly being performed to test the resilience of life forms in either vacuum chambers or real outer space, courtesy of the International Space Station. After all, if terrestrial life can survive in such a hostile environment, the higher the likelihood that alien microbiology could arrive on Earth via meteorite impact or cometary tail (and at least one amino acid, glycine, has been found on several comets).
Unmanned probes are now replicating these findings, with the European Space Agency's Rosetta spacecraft finding glycine in the dust cloud around Comet 67P/Churyumov-Gerasimenko in 2016. Although these extra-terrestrial objects may lack the energy source required to kick-start life itself, some are clearly harbouring many of the complex molecules used in life on Earth.
It has now been proven beyond any doubt that organic and pre-biological material is common in space. The much higher frequency of impacts in the early solar system suggests that key components of Earth's surface chemistry - and its water - were delivered via meteorites and comets. Unfortunately, the unwary publication of provisional results, when combined with the general public's feeble grasp of scientific methodology, has hindered support for what is surely one of the most exciting areas in contemporary science. A multi-faceted approach may in time supply the answers to the ultimate questions surrounding the origin of life and its precursor material. This really is a case of watch (this) space!
