Stephen Jay Gould
Is evolution universal: would evolution on other worlds
operate similar to Darwinian evolution?
The problem with the history of life is that we only have one
experiment. All life on earth, by virtue of great biochemical
similarity, we assume, came from one origin, and therefore the fact that
all living things are similar on this planet gives us no insight
whatsoever into the crucial question of whether life has to be organized
the way it is or whether what we see on earth is only one possibility
among a hundred million unrealized alternatives.
I don't really know any way to get at that question except the
experimental hope that we'll someday encounter living systems that
independently evolved on other worlds. That's why the question of
whether or not life exists on that Martian meteorite raises such
legitimate excitement. It probably doesn't, although the case is
plausible. What we're desperate for is a second independent experiment
in the origin of life; then we can draw some of those conclusions.
Without that, it's all speculation.
If you had to speculate about a second example of life, of
extraterrestrial life, would you expect commonalties?
I have tried to argue that unpredictable contingency rules the
pathways of history in that prediction is a subject we shouldn't even
entertain much for the history of life. And I'm going to stick with
that. I really don't think there's a whole lot of very great interest
in any detail that one would want to predict about life on other worlds.
The oldest rocks on earth that could contain organisms are 3.5 to 3.6
billion years old, and they do have life in them, so life on this planet
originated about as quickly as it could. At the most, I might be
willing to say that you could make an inference that, given planets like
the earth and given conditions such as we find on earth that there
may be commonalties, given the nature of organic chemistry and the
physics of self-organizing systems being a predictability if not a
virtual inevitability of the origins of some form of living creatures.
That's a very different subject from what evolutionary pathways they
take later. We need to get direct evidence; all the more reason to hope
that some day we'll have evidence of life elsewhere.
Rather famously, you've compared life to a bush rather than a
tree. How does the evolutionary bush look?
The reasons I was emphasizing bushes as opposed to trees is at
least certain trees have, as their anatomy, a very strong central trunk
and a bunch of side branches. That leads to undue and unfortunate
support for the rather biased and prejudicial notion that the history of
life has a central directionality represented by the main trunk of the
The reason why I think you ought to think in terms of bushes is that if
anything's characteristic of life, it is its multifariousness, it's
diversity. A bush doesn't have a central fundamental pathway, it just
has a common root. You do need to have a common root source, as
evolutionary theory does insist on common ancestry for related living
things. But once you have that common root, the bush can go off in
hundreds of directions with no preferred pathway. That's why I think
it's a better metaphor or better model for depicting the history of
What's missing from the Darwinian view of things? Do we now
understand evolutionary principles completely?
I think we certainly have developed and validated some very
basic, general and, I think one can say, true principles that regulate
evolutionary change, of which natural selection is the most important.
But we certainly don't have anything like a complete explanation or a
complete catalog of evolutionary mechanisms.
As a paleontologist, I think what's most lacking is adequate theory to
explain the full range of large-scale events in the three and a half
billion years of life's history. It's not clear to me that you can take
Darwinian processes operating in small scale and, just by extrapolating
them through hundreds of millions of years, in every case explain
everything about the pattern of life as just ordinary processes
operating on a weekly or monthly or yearly scale extended.
The most obvious example, of course, is mass extinction. That is, the
history of life is patterned to a large extent by these very brief
episodes, some of them catastrophically triggered such as the one that
wiped the dinosaurs out 65 million years ago. That was pretty clearly
instigated by impact from an extraterrestrial object. These mass
extinction have occurred at least five times in the 500 million year
history of complex animal life. They play a major role in helping us to
understand the waxing and waning of groups and why some groups dominate
and others don't.
So if you want to ask, for example, why mammals are here and dinosaurs
are gone, the answer is not going to come out of ordinary Darwinian
competition and the daily struggle for existence, because it all
happened as a result of this large extraterrestrial body that struck the
earth 55 million years ago, and for reasons we don't fully understand,
wiped dinosaurs out and mammals managed to survive. It was not, one
assumes, for any reason of their inherent superiority in competition
against dinosaurs, because, in fact, they had competed against dinosaurs
for 100 million years before that and had never displaced them.
The pattern of life's history is a complex mixture of who wins and
survives an ordinary Darwinian competition extended in normal times, and
the fortuity of who does or doesn't get through these mass extinctions
for which no one can prepare. To that extent, conventional Darwinism
can't explain all of the pattern of life's history. You need to
introduce into it an understanding of what happens in the larger scale
What are your thoughts about intelligence as a glorious
I don't think anyone would doubt that the gap between what humans
uniquely do and any other species does is immense. It's based on
language, it's based on the development of technology. Just in terms of
impact on the surface of the planet, I think that's evidentI don't
think that's just human arrogance.
The main reason for considering intelligence in that efficacious form
(and I don't necessarily mean good) as accidental is pretty clear. It's
only evolved in one species after four and a half billion years of the
history of the earth, which is about half the earth's potential history
if the sun's due to explode in five billion years or so. That's pretty
amazing. There are only 4,000 species of mammals. We're a minor group.
Consciousness has only arisen in one species, us, of a minor order of
mammals: the primates with fewer than 200 species in toto. You have a
million named species, including about 500,000 beetles. So if
intelligence was such a good thing , so obviously a Darwinian benefit,
and easy to achieve, I assume other lineages would havebut they
haven't. And yet they're doing very well. I think the best evidence of
the accidental nature of intelligence, is the very sparse distribution
of this phenomenon after three and a half billion years of life, and
four and a half billion years of the earth's history.