Where Did Eyes Come From?

ERIC: Imagine standing on a beach half a billion years ago. The land behind you is completely devoid of life. It hasn't yet crept out of the oceans, which are teeming with a wide variety of plants and animals.

Some of that might look not very out of place in today's oceans and some that look bizarre enough to have come from another planet.

Today on Pulsar, we're answering your questions about one of the life forms in that ancient ocean, trilobites, and what their fossils can teach us about evolution and the origins of sight.

I'm your host, Eric. Thanks to Facebook Boston for supporting this episode of Pulsar.

Today my guest is joining me all the way from the University of Cologne in Germany.

BRIGITTE: I'm Brigitte Schoenemann. I'm a zoologist, and I do my research work here on trilobites and trilobite vision.

ERIC: Dr. Schoenemann, thanks so much for calling in to the podcast. Let's start with some basics. What is a trilobite?

BRIGITTE: Yes, trilobites are extinct invertebrate animals that lived in the ancient seas. All trilobites were marine. They mainly lived in freshwater or terrestrial.

We find them from the beginning of the Cambrian some 520 million years ago up to the Great Dying at the end of the Permian 251 million years ago.

So, yeah, I suppose like today's shrimps, crabs, lobsters are having a hard external skeleton and jointed legs with which they could walk around on the sea floor or swim above it.

They became highly diverse and abundant colonizing many different marine environments. The class trilobites contains 9 orders, 150 families, 5,000 genera, and more than 20,000 species.

ERIC: So they were a very successful form of life a long time ago on the earth. What made them so successful?

BRIGITTE: Oh, I think they were so successful because they were cosmopolitans. They were diverse and very flexible.

ERIC: Well, we see that diversity and flexibility in many species today, but trilobites were one of the first super successful animals. Can you tell us when they lived on the earth?

BRIGITTE: So all these trilobites are of lower Cambrian age occurring some 522 million years old, and they ranged through the whole of the Paleozoic finally becoming extinct in the late Permian about 250 million years ago.

This was not because they were biologically inferior. They were highly successful, quality lifeforms that survived successfully for 272 million years.

They were at their most diverse during the Ordovician, about 450 million years ago. It came through several mass extinctions and severe environmental crises, and invaded most different ecological niches, from the planktonic to the deep sea.

Finally, they became extinct because of successive environmental catastrophes, to which they were no more immune than we are.

ERIC: Wow, 272 million years is a long time. Katie asked us, "What was the earth like when trilobites existed, and how is it different from today's earth?"

BRIGITTE: The world they inhabited was our world. But the continents moved around, collided, and merged, and only the North American continent stayed in the same position throughout the Paleozoic. There were both long-term and short-term climatic fluctuations.

There were glaciations, toxic episodes, and ocean chemistry, colossal volcanic episodes, and major changes in sea level.

Great reefs grew and died, and all these changes affected the oceans in very many ways, having substantial effects on evolutionary changes to trilobites. And our trilobites dominated the faunas of these oceans. During the Paleozoic, it was much warmer than today, several times interrupted by ice ages.

ERIC: Well, it sounds like they were able to adapt and survive, despite these changing and extreme conditions for so long. But other animals must have been able to as well. So why is it that we find so many trilobite fossils in particular?

BRIGITTE: So we have seen the trilobites dominated the fauna of their time, and were very abundant. When they died, the material of their shells sometimes became reinforced or substituted by minerals, especially calcite, which the skeleton already contained.

Trilobites are preserved in many kinds of marine sediment, especially limestones, sootstones, mudstones, and shale.

The hard external skeleton was made of calcite, set in an organic base. Whenever a trilobite died and was buried in the lime mud sediment, it would very likely be preserved without much, if any, change, and with all its surface features intact.

The delicate legs and other such organs are only preserved under especially favorable conditions.

ERIC: Now, you study the eyes of these ancient creatures, and the origins of sight in animals. So where did eyes come from? Why did animals first evolve them?

BRIGITTE: Yes it's interesting. In the times before the Cambrian, there was not enough oxygen in the water to allow greater organisms to live. Creatures of that time were small and flat, and glided over the floor of the seas, feeding on algae mats.

It was a peaceful world. There were no predators, no prey, and there were no eyes. The race of arms to see and to be seen began in the Cambrian with its sometimes ferocious predators coming up.

ERIC: So some organisms evolving sight in order to avoid predators, while the predators themselves needed sight to find their prey. It occurred to me after reading about some of your work that we don't usually see eyes in the fossil record. How can a trilobite fossil tell us about their vision?

BRIGITTE: In order to fossilize, animals or their organs need to have hard parts, like bones or shells, and need to be buried rapidly. If we are thinking about human or vertebrate eyes, one would not expect that they could ever be fossilized.

But part of eyes of the trilobites, like the rest of the exoskeleton, were made of calcite. They eyes of trilobites are compound like those of insects, crabs, and lobsters.

But whereas a compound eyes of living arthropods are made of organic material, the lenses of trilobites were constructed of calcite, which has a virtue of being transparent.

So it has somewhat different optical properties. The lenses are normally only part of the eye to be fossilized. The critical light receptive organs below, upon which the lenses focus light, are only very occasionally preserved.

ERIC: Well, that's incredible that we can use these fossils to learn about sight from so very long ago. How do you actually go about studying the eyes themselves?

BRIGITTE: The external structure of trilobite eyes is very evident. From the very beginning of their appearance in the early Cambrian, they showed the lattices of a faceted eye, very much comparable to the compound as of modern insects and crustaceans, often with an enormous field of view all around.

So they can look forward, backwards, and especially to the side, scanning the horizon. Strangely, many of them cannot look upwards. But the most important other internal structures which tell us how the eye actually worked. The structure of these old eyes could be anything.

But applying modern techniques such as synchrotron radiation and X-ray tomography, it has been possible to trace other relics of the cellular equipment, and thus the sensory functions.

ERIC: So you're using technology that wasn't available to previous generations of scientists studying trilobites. What have you learned in your research?

BRIGITTE: We know now that most trilobite eyes, in principle, worked like the compound eyes of many modern-day active arthropods. In fact, each facet does not form an individual picture of the environment, but contributes just one point, like a pixel of a computer graphic.

So among other factors, the number of pixels, facets, is responsible for the acuity of vision.

Another point is that generally large facets can catch more light and smaller ones. So smaller facets can work well under bright light conditions, but not in the dawn, for example.

Consequently, crepuscular, nocturnal, or deep-sea arthropods need wider facets to capture as much light as possible.

So actually, even a fossil can tell you which details of the environment the ancient creature could see, which range of the surrounding, and even whether it lived in light flooded water, shallow water, or close to the water surface, or whether it preferred dim light conditions, early or late in the day, or lived in deeper regions of the sea.

ERIC: Well, it's really amazing that we can use the fossilized structures of these creature's eyes to determine how they lived. But also, to learn that they saw in a very similar way, as some modern day animals half a billion years later.

And so to wrap up, thinking about the bigger picture, what can studying trilobites and their vision tell us about the evolution of eyes?

BRIGITTE: I think that the most important is that we can observe over more than approximately 270 million years of time the development of eyes, and that it has been a system so stable that it must be really a good and effective system.

ERIC: Well, Dr. Schoenemann, thank you so much for calling into Pulsar, and sharing your research.

BRIGITTE: Oh, thank you. Have a good day.

ERIC: To learn more about ancient life on Earth, tune in Our Ask A Scientist live streams about dinosaurs on mos.org/mosathome.

Until next time, keep asking questions.

Theme song by Destin Heilman