Yes, these are the days I really miss Stephen Jay Gould's writing. I
frequently post to this list things that I find in the mainstream media
that intrigue me and solicity more information about it, because I
sometimes find the implications difficult to understand. Although they
quote from Gould here, I can't make head or tale out of the metaphor
they're using about evolution stumbling out of a bar onto a train
platform and the train tracks, all symbolizing, they say, some features
about evolution. I'd skip over it, usually, but because this seems
interesting I'm posting it here for insight, especially from Stuart
Newman and the other scientists here ....
Evolution: That Famous ‘March of Progress’ Image Is Just
New research shows animal evolution often involves losing
genes and becoming less
by Jordi Paps
Photo from Usagi-P / Shutterstock.
Evolution explains how all living beings, including us, came to be. It
would be easy to assume evolution works by continuously adding features
to organisms, constantly increasing their complexity. Some fish evolved
legs and walked onto the land. Some dinosaurs evolved wings and began to
fly. Others evolved wombs and began to give birth to live young.
Yet this is one of the most predominant and frustrating
misconceptions about evolution. Many successful branches of the tree
of life have stayed simple, such as bacteria, or have reduced their
complexity, such as parasites. And they are doing very well.
In a recent
study published in Nature Ecology and Evolution, we compared the
complete genomes of over 100 organisms (mostly animals), to study how the
animal kingdom has evolved at the genetic level. Our results show that
the origins of major groups of animals, such as the one comprising
humans, are linked not to the addition of new genes but to massive gene
The evolutionary biologist Stephen Jay Gould was one of the strongest
the march of progress”, the idea that evolution always results in
increased complexity. In his book
House (1996), Gould uses the model of the drunkard walk. A drunkard
leaves a bar in a train station and clumsily walks back and forth over
the platform, swinging between the bar and the train tracks. Given enough
time, the drunkard will fall in the tracks and will get stuck there.
The platform represents a scale of complexity, the pub being the lowest
complexity and the tracks the maximum. Life emerged by coming out of the
pub, with the minimum complexity possible. Sometimes it randomly stumbles
towards the tracks (evolving in a way that increases complexity) and
other times towards the pub (reducing complexity).
No option is better than the other. Staying simple or reducing complexity
may be better for survival than evolving with increased complexity,
depending on the environment.
But in some cases, groups of animals evolve complex features that are
intrinsic to the way their bodies work, and can no longer lose those
genes to become simpler - they become stuck in the train tracks. (There
are no trains to worry about in this metaphor.) For example,
multicellular organisms rarely go back to become unicellular.
If we only focus on the organisms trapped in the train tracks, then we
have a biased perception of life evolving in a straight line from simple
to complex, mistakenly believing that older lifeforms are always simple
and newer ones are complex. But the real path to complexity is more
Together with Peter Holland from the University of Oxford, we looked into
how genetic complexity has evolved in animals. Previously,
we have shown that the addition of new genes was key to the early
evolution of the animal kingdom. The question then became whether that
was the case during the later evolution of animals.
Studying the Tree of LifeMost animals can be grouped into
major evolutionary lineages, branches on the tree of life showing how
the animals alive today evolved from a series of shared ancestors. In
order to answer our question, we studied every animal lineage for which a
genome sequence was publicly available, and many non-animal lineages to
compare them against.
One animal lineage is that of the deuterostomes, which includes humans
and other vertebrates, as well as sea stars or sea urchins. Another is
the ecdysozoans, comprising the arthropods (insects, lobsters, spiders,
millipedes), and other moulting animals such as roundworms. Vertebrates
and insects are considered some of the most complex animals. Finally, we
have one lineage, the lophotrochozoans, that includes animals such as
molluscs (snails, for example) or annelids (earthworms), among many
We took this diverse selection of organisms and looked to see how they
were related on the tree of life and what genes they shared and didn’t
share. If a gene was present in an older branch of the tree and not in a
younger one, we inferred that this gene had been lost. If a gene wasn’t
present in older branches but appeared in a younger branch, then we
considered it a novel gene that had been gained in the younger branch.
A tree of life diagram showing the changing number of
genes of different animal groups. Downward pointing orange triangles
indicate gene losses. Upwards pointing green triangles indicate gene
gains. The bigger the triangle, the greater the change. Credit: Jordi
Paps, author provided.
The results showed unprecedented numbers of genes lost and
gained, something never seen before in previous analyses. Two of the
major lineages, the deuterostomes (including humans) and the ecdysozoans
(including insects), showed the largest number of gene losses. In
contrast, the lophotrochozoans show a balance between gene novelties and
Our results confirm the picture given by Stephen Jay Gould by showing
that, at the gene level, animal life emerged by leaving the pub and
making a large leap in complexity. But after the initial enthusiasm, some
lineages stumbled closer to the pub by losing genes, while other lineages
drifted towards the track by gaining genes. We consider this the perfect
summary of evolution, a booze-induced random choice between the bar and
the train track. Or, as the internet meme says,
go home evolution, you are drunk.”
Jordi Paps is a Lecturer at the University of Bristol’s School of
Cristina Guijarro-Clarke is a PhD Candidate in Evolution at the
University of Essex.
This post originally appeared on The Conversation and was published March
2, 2020. This article is republished here with permission.