Mystery worm sheds light on ancient evolution

Creepy crawler: Research has literally turned the Hallucigenia on its head © Danielle Dufault

It has taken scientists 100 years to understand the basic anatomy of a small extinct sea creature, after several scientific mistakes. Can we ever be sure of our scientific knowledge?

It may look like something from an alien movie, but 500 million years ago, the Hallucigenia was one of the world’s most common creatures. Its strange appearance has baffled scientists for decades, but they have now come one step closer to understanding the sea worm’s secrets.

In 1977, a paleontologist named Simon Conway-Morris rediscovered a strange fossil that had originally been found in the Burgess Shale in 1909. It had been categorised as an early relation of today’s leeches and earthworms, but Conway-Morris realised that this was false. He named the creature ‘Hallucigenia’ because of its strange appearance. It is just half an inch long, and he believed it had walked on seven spikes which acted as stilts, with seven tentacles waving on its back.

In the early 1990s, related fossils were discovered in China which disproved this theory. The spikes were in fact on the creature’s back; Conway-Morris had got the creature upside down.

Despite this discovery, one major question remained — which end was which? After studying the fossils under new electron microscopes, Dr Martin Smith at Cambridge University finally discovered ‘not just the pair of eyes, but also this big smile of teeth grinning back at us.’

The discovery tells us more about the creature’s place in evolution. It is part of the ‘Cambrian explosion’, a period during which there was a sudden burst of new diversity among animals. Some of these ‘evolutionary experiments’ included chordates, animals with a dorsal nerve cord — nowadays these include fish, mammals and humans — as well as brachiopads with hard shells like clams, and early ancestors of insects.

The Hallucigenia’s teeth are particularly surprising: its closest relative on earth, the velvet worm, is toothless. The Hallucigenia suggests that teeth appeared and later disappeared somewhere along the evolutionary line. ‘Evolution is a gradual process,’ said Dr Smith. ‘By deciphering “in-between” fossils like Hallucigenia, we can determine how different animal groups built up their modern body plans.’

Know limits

We like to think of scientific knowledge as something solid that we can rely on, which can explain the world we live in. But the Hallucigenia’s story is full of mistaken conclusions and unanswered questions. Some may say that it is frightening to think that what we believe we know about the world may not be true.

But this misunderstands the nature of science, others point out. It is a discipline which is always correcting itself and expanding its horizons further. The boundaries of knowledge are limitless, and the constant revisions make it exciting. You never know what may be discovered next!

You Decide

  1. Is the Hallucigenia any more strange than the life forms that still exist today?
  2. ‘Science is the only way of gaining reliable knowledge about the world.’ Do you agree?


  1. Imagine conditions on planet Earth changed rapidly over the next 100,000 years — for instance, the planet became much more hot. Write a paragraph or draw a diagram imagining how one animal might adapt to the changes.
  2. Research an animal (extinct or otherwise) that you find particularly bizarre. Write a brief presentation explaining how it evolved the way it did.

Some People Say...

“Science advances one funeral at a time.”

Max Planck

What do you think?

Q & A

Why are scientists so preoccupied by a creature that died 500,000 years ago?
Partly out of sheer curiosity: it’s such an odd-looking animal that uncovering what it looked like and how it interacted with the world has become a sort of scientific puzzle. But learning about ancient organisms also has broader applications: it enables us to trace how genetic characteristics evolve, how species are related to each other and how Earth’s current inhabitants came to be the way we are.
Might scientists change their mind about the Hallucigenia again?
Thanks to new microscope technology, the researchers are pretty confident that they’ve got at least the essentials correct this time. But there could still be tweaks and refinements. No good scientist claims to have solved a problem definitively!

Word Watch

Burgess Shale
This area of the Rocky Mountains in Canada is exceptionally rich in a huge array of ancient fossils. When the first of these were discovered in 1909, they were so diverse and so alien from modern life that they revolutionised our understanding of how life evolved. Many of the fossils found here have no living descendants.
Electron microscopes
Microscopes that work by firing electrons at a specimen to illuminate it. Until recently, the specimen needed to be sprayed with gold particles, causing irreparable damage. Now all that’s needed is water.
Cambrian explosion
Until about 540 million years ago, most Earth’s life forms were relatively similar. Then, over a period of around 25 million years — a blink in evolutionary terms — a burst of diversification gave rise to the ancestors to which almost all of today’s species were born. Scientists are still discovering the reasons for this explosion, but theories include a rise in atmospheric oxygen and a sudden acceleration in the ‘arms race’ between predators and prey.


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