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 ....

Thank you.


Evolution: That Famous 'March of Progress' Image Is Just Wrong

New research shows animal evolution often involves losing genes and 
becoming less complex.

<>The Conversation

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 
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 losses.

The evolutionary biologist Stephen Jay Gould was one of the strongest 
opponents of 
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 tortuous.

Together with Peter Holland from the University of Oxford, we looked 
into how genetic complexity has evolved in animals. Previously, 
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 Life

Most animals can be grouped into 
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 others.

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 losses.

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, 
home evolution, you are drunk."

Jordi Paps is a Lecturer at the University of Bristol's School of 
Biological Sciences.

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.