I tend to highly agree with this statement, but as I thought about it more I am not so sure.
My thoughts are that we do not know what we don't know. If the life evolved on a planet similar to ours I think this is very likely.
Even though its a bit of a misnomer, I added that "
highly" evolved qualifier to supply some implicit assumptions that we're dealing with an environment that would foster a lot of change in the inhabiting species. It'd be a bland discussion to consider a planet where only simple bacteria evolved. I assumed we're more along the Star Trek line here where we're talking about species that would be interesting to us as humanoids.
But there is the rub. We have no idea of what alien life could be like. Is bilateral symmetry a result of unique circumstances in the evolution of life on this planet?
I think that bilateral symmetry did evoke an important advantage that has been utelized ever sense it emerged. I don't think there are any species that subsequently lost it. And, again considering things that would be interesting to us as humanoids, almost all of the Animals exhibit bilateral symmetry.
I think there is a very high likelihood that other life out there exhibits bilateral symmetry, but am very open to the idea that we may find something that utterly astounds us and falls completely outside of any of our expectations.
See, I just don't thing we find some really wacky alien that lacked any symmetry and was just totally bizarre in that regard (at least, talking about something on par with the Animal clade). I'd expect symmetry.
To put it simply, with the knowledge we have I think you are correct. It is what we don't know that is the wild card.
Sure, but what kind of enviroment could exist that would favor assymetry and still push towards something like Animals?