Charley Lineweaver, a scientist with the SETI Institute, argued today that pure human vanity, not scientific evidence, leads us to believe that if humans were wiped off the face of the Earth some other species (probably an ape of some kind) would rise to fill the "intelligence niche" that we Homo sapiens currently occupy. He calls this the "Planet of the Apes hypothesis," and believes that life here on Earth has already shown it to be false. That means the way we're searching for extra-terrestrial life — or even the act of searching itself — is terribly misguided.
Lineweaver's idea kind of rocks SETI scientists' mission statement to the core. Ever since Carl Sagan's famously framed the ET question "are we alone?" as "Are there functionally equivalent humans elsewhere in the universe?" SETI folks have been trying to answer it. It's a gargantuan task, and one that that Lineweaver argues we're making worse by assuming that there is something about humans that is unique or special, or that life on Earth "wants" to be human.
If there is any tendency for life to evolve to get as functionally human-like as possible, then Lineweaver asks why haven't isolated part of Earth evolved human-like intelligence? Madagascar has been separated from Africa for millions of years, and should therefore be full of high-level primates instead of lemurs — apes' distant cousins. New Zealand (which because of its isolation Jared Diamond said was "the best opportunity we'll ever have to study life on another planet") should be filled with super-intelligent giant birds.
Lineweaver thinks that big brains aren't the be-all and end all of evolution. In fact, he argues that the answer to Sagan's question is "no" — functionally equivalent humans don't exist elsewhere in the universe. Instead, life elsewhere might be so weird as to be unrecognizable. "Intelligence" could easily take the form of some kind of system at profound disequilibrium with its environment — something like a hurricane or a star could be intelligent.
It's sounds like he's begging to get the SETI Institute's funding pulled, and to declare the entire SETI operation utterly useless, and in a sense he is. But he also thinks it's worth continuing the search because there's a lot of unexplored universe still out there to look at. And he admits he could be wrong — there could be a Planet of the Apes out there, too.
Lineweaver presented his theories at the Astrobiology Science Conference 2008.