The March of Progress, Rudolph F. Zallinger. From Time Life's book Early Man.

The so called "march of progress" has been used to describe the origin of our species thousands upon thousands of times. But it never happened. Only a few of the species depicted are potential ancestors for humans and many of them were contemporaries to each other (as the original diagram makes clear) so can hardly be different steps along a single evolutionary path. 

Quite some transformation. Needless to say, amphipods are not prawns and the creature in the larger photograph is only distantly related to the mega-amphipod in the smaller one.

Am I being ridiculously pedantic to care about this error? Is my life really so easy that I have time to worry about taxonomic and nomenclatural details associated with reports in the Herald? In fact, there's more to this error than the name. The Herald's conflation of these two distantly related groups highlights the way we fail to comprehend the true diversity of life on earth. So here's my attemp to show what you miss when you call an amphipod a prawn.

The scale of biodiversity

Every cell in my body is the result of an unbroken-chain of cell division that stretches back 3.8 billion years. I find that an absolutely staggering fact, but I shouldn't feel too special. Really, I 'm just like every creature on earth - riding on the edge of one of the lineages that extends out from biology's big bang at the origin of life. Plenty of people have tried to illustrate the history of life, here's one more look, based on DNA sequences from living organisms:



Forget animals and plants as the "two kingdoms" of life. For most of the history of life every creature on earth was a single cell. Even today, most of the lineages that extend out from the biology's big bang are made entirely of singled-celled organisms. Because this tree is based on those species that scientists have sequenced the genomes of, it drastically over-represents animals, but even here all animals are represented by the little twig that descending from the black-labeled node.

Biodiversity is more or less fractal, if we zoom in on that node and look at the phylogeny of animals we get another tree with branches:


As I've said before, it's very difficult to resolve the relationships among the different lineages of animals. No matter how the branches relate to each other, it's clear there are a lot of different ways to be an animal, and that I'm never going to run out of subjects to write about for these Sunday posts. All the lions, tigers, bears, elephants, fish, fowl, lizards and frogs that make their way around the world with the aid of a backbone are part of the branch labeled Chordates above. The rest are invertebrates, and so fair game for me. But I'm meant to to be talking about amphipods, so let's skip a couple of steps and move into the arthropods (jointed-limbed creatures including insects) then down further into crustaceans and in particular the class Malacostraca - a group that includes crabs, lobsters, shrimps, wood lice and indeed amphipods and prawns:

Now we can start to how broad a brush you'd need to paint amphipods into the same group as prawns. In fact, the lineages that went on to give rise to modern amphipods, on the one hand, and prawns on the other parted ways about 130 million years ago. That's much more ancient than the divide between, say, mice and elephants. What's more, the amphipods are a much more diverse and interesting group than the prawns into which they were lumped. 

Caprella mutica. Photo is CC-BY-SA 3.0 from Hans Willewart

The bumps around the heads of right whales (called callosities) are part of the whale's body, but the white colouration results from the thousands of cyamid amphipods which live on these bumps. Unlike barnacles, which also attach themselves to whales, cyamids don't have a free-swimming larval stage, so they are only passed on from one whale to another through contact. Researchers have even used the genetic relationships between cyamids to learn about relationships among whale populations

Cyamids are commonly called "whale lice", and indeed their flattened bodies are similar to those of their distant insect cousins:

Photo CC 2.0 BY-NC from Flickr user J Whelcher

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By contrast with the diversity found within the amphipoda, prawns are pretty limited morphologically. I realise the Herald's headline synonymising the first into the second reflected a general lack of understanding about the sorts of creatures that fill our biosphere, rather than a particular oversight on their behalf. But when the true picture is so much more interesting than the lazy shorthand they used, I think it's worth pointing out the error.