Probably - That's Why Planetary Protection Guidelines Need to be Enforced
Cyanides appear to have played a significant role during the origin of life. According to a new study, they also helped establish a critical metabolic pathway for early life on our planet. However, most of us know cyanides from a different side: as highly toxic compounds produced by many different species, including bacteria and fungi. Cyanide consists of the biogenic elements carbon and nitrogen, which is poisonous to most animals, because it interferes with both aerobic respiration and ATP synthesis. Thus, cyanide is a good example of being a potent toxin but also having beneficial aspects to life.
In Earth’s biosphere, the use of a broad range of venoms is well known for various species both for defense purposes and catching prey. There are alone more than 200,000 venomous animal species. So, would we expect the ubiquitous use of toxins also in an alien biosphere?
The path that evolution takes can vary a lot. This becomes evident when we compare the fauna of Australia and New Zealand, two land areas that are located relatively close to each other. Australia has some of the most venomous animals on our planet. It is home to 20 of the 25 most venomous snakes in the world, including the top ten (the most venomous one, the Inland taipan, has enough venom to kill 250,000 mice in a single bite!). Compare that to New Zealand, where there are no venomous snakes or scorpions at all, just one venomous native spider (the katipō). Nature clearly took a very different approach in New Zealand on using venom.
In the rest of the world, toxin is not as drastically utilized as in Australia, but still pretty common. We see this in many plants, which can be fully appreciated by visiting Alnwick Garden in Northumberland (UK). The use of toxins in so many unrelated species can be seen as a result of convergent evolution, which is the independent evolution of similar features or traits in species adapting to their changing environment. Simon Conway Morris described this process in his book Life's Solutions: Inevitable Humans in a Lonely Universe very well.
Convergent evolution occurs not only on Earth but would also be expected to occur in an alien biosphere. I would go as far as to suggest that any technologically advanced aliens would have some of the same traits as we humans have. Examples could be a large brain, dexterity of their appendages (such as hands or tentacles), and being social.
Would intelligent aliens be venomous? It is more likely not, because intelligence is energetically costly, and natural selection would favor its evolution over venom. An example is snakes: it is more advantageous for them to be fast, have a firm grip on their prey, and inject a strong venom. It is not important for snakes to be very smart. However, there can be exceptions. Octopuses are highly intelligent, but some of them use a strong venom, such as the blue-ringed octopus, which applies the neurotoxin tetrodotoxin. Also, there are some venomous mammals, the most being the platypus (from Australia, too – how could it be otherwise?).
Interestingly though, the organism that produces the most toxic substance known is an unassuming and relatively primitive bacterium, Clostridium botulinum, which thrives under oxygen-poor conditions. Already at tiny concentrations, its toxin, Botulinum, blocks nerves functions and leads to respiratory and muscular paralysis in humans. One bean contaminated with this toxin can easily kill you (the lethal dose is about one nanogram (one-millionth of a gram) per kilogram bodyweight!).
Based on the Cosmic Zoo hypothesis, we expect that there are many alien biospheres in the Universe, some of them complex with animal and plant-like creatures. Once we are ready to explore these, we should proceed carefully. More importantly, we should be cautious when bringing biological samples back to Earth. This also pertains to Mars, where we do not expect more than a limited microbial biosphere. In any scenario, there is a chance that we return organisms from an alien planet back home. Therefore, planetary protection guidelines should be strongly enforced.