How Light Became Sight: All About Color, Part V

Logo for our hypothetical time travelers

Imagine with me for a few moments that mankind has mastered time travel, and that we are members of a scientific expedition to a time 700 million years in the past. We stand in our heavily protected spacesuits on a small sandy beach along a strip of rocky coastline beside a shallow arm of a much larger sea. A plume of smoke rises from a volcano on the far horizon. We need the spacesuits because the atmosphere is thin and doesn't give much protection from the planet's sun. We cannot breathe the air of this planet, because it is mostly nitrogen and carbon dioxide. Despite the light reflected from the coarse sand, our thermometers show an uncomfortably chilly temperature. We will have to work fast, because the days are considerably shorter than we are used to. We have come to study the alien life on a planet far stranger than Mars.

Welcome to the Earth.

Underwater ROV

After a short session of directions from the team leader, including that all specimens must be enclosed and sealed immediately and that their surfaces as well as our suits will go through a decontamination process before returning to our own time. Scientists scatter to locate promising collection spots. They march across the narrow, orange-tinted beach toward low, darker orange rocks. You and I remind the scientist who is preparing a probe to the top of the atmosphere that we will need a copy of her data, and walk to the place where a robot submersible is being deployed.

Depth of penetration for colors

The operator describes how the machine will first make a circle 2 kilometers wide on the surface and then descend in concentric circles at 10 meter intervals until it reaches the bottom, where it will take samples. It will collect live specimens as we direct as it makes its circuits. We explain that we need measurements of the light entering the water at each level, both in intensity and color. This is set up for automatic measurements and we settle down in front of our monitors, told to "sing out" when we see an organism to be collected. We advise him to watch for "cloudy" or colored patches which indicate concentrations of microscopic life.

Cyanobacteria (field sketch)

The water is surprisingly clear, with a distinctly green color.  We have already heard an excited scientist reporting a pool with blue-green algae (actually bacteria) on our intercoms, so they were certainly in the water as well. Moments later the submersible secured a specimen of the "blue-greens" for us. We could later compare it with the specimens from the pool as well as with ones from our own time.  As the hours pass, we direct collection of thin films of red and green organisms, hoping the green might be an early plant, and anything unusual in the water. We know that we are dealing with organisms far too small to see even with the magnifying lenses on the submersible's probes, plus the ones on our monitors.

Sponges

As we descend,  the colors fade and the light grows dimmer. We are excited to encounter a colony of small sponges — the first multi-cellular creatures we have seen. We gather both the organisms that move toward us and the ones that move away. One group about the size of marshmallows appears red, a color that would not show up against the darkness of the water below 200 meters. They move away from our lights, quickly, followed with a flash of blue luminescence. A probe swings out to collect some of them.

Lichen

A warning from the submersible indicates that it is approaching the sea bottom. Additional floodlights come on and the submersible hovers close to a vertical wall of rock. A number of samples of the sea bottom are taken, separating the levels of sediment. We get a call warning that we should return to base and have our specimen boxes ready for decontamination within the next half hour. The submersible rises at an angle to take it increasingly closer to its launch point. We secure our monitors, sorry to bring our adventure to an end. We dock to find other scientists ready to help us get our specimen boxes decontaminated and secured and then line up for the decontamination of our suits.  People are chatting about their finds. "There 's a lot more oxygen in the atmosphere than we expected," the atmospheric specialist tells us. "The ozone layer may already be forming." The man who found the blue-green bacteria is disappointed that he didn't find lichens. We tell him that the ultraviolet light is probably still too strong.

Well, that was fun, but what does it have to do with light, color, and sight?
 

Some squid are benthic night feeders

First of all, let's look at the chart above showing how colors of light disappear as they penetrate water. This phenomenon holds true in deep, relatively still water anywhere on the planet. You can see that the color red disappears very quickly. Up to that depth everything looks pretty much like it does above the surface. Only a few meters further and a red or orange fish would look almost black. (Sediment, algae, debris or turbulence would reduce visibility still further.)  Eventually, you would need powerful lights to see anything at all.  Even those would not penetrate the darkness to a very great distance. Yet organisms live at great depths.  Don't you wonder how? Many benthic (deep water)  animals come to the surface at night to feed or spawn. Some of them have eyes and some do not.

Algal bloom on Lake Erie

The team member's finding of blue-green bacteria was significant. Very soon after the oceans formed on young Earth, these bacteria developed the ability to use sunlight to make food using carbon dioxide (photosynthesis), releasing oxygen as a by-product. For a long time, the oxygen was absorbed by different kinds of rock, but eventually it began to replace some of the carbon dioxide in the Earth's atmosphere. It still does. Now we could observe the process and see how, or even if, it had changed.  These cyanobacteria live in most lakes and are part of a lake's normal ecosystem. Unfortunately, high nutrient concentrations can cause "population explosions" and a "bloom", especially in warm weather.

Our Sun

The information from the atmospheric scientist was also quite important. It meant that the atmosphere was soon going to be able to support many more kinds of organisms. It also hinted that an ozone layer might already be forming at the top of the atmosphere. The sun delivers three kinds of light: visible light, infrared light (heat), and ultraviolet light. Too much ultraviolet tends to damage or kill cells, as you know if you have ever had a bad sunburn. The ozone layer would stop a large portion of the ultraviolet from reaching Earth's surface. This favored many multi-cellular organisms, while it caused other species to become extinct. Today these "blooms" are altering water quality in bodies of water like the Great Lakes, fouling the water and air.

Electron microscope

Now let's look at those specimens we collected.  These sketches represent fossils of known microscopic organisms or modern species which are believed to have an ancient ancestor, which they resemble.) Each specimen is shown as it would look under high magnification. The fossil record of such organisms has been greatly expanded and improved in the past few years with the use of electron microscopes.

"Red tide"

I try to keep up with science news as much as possible. Recently I saw a sentence that really caught my attention. It said:"The dinoflagellates are not happy." No, they're not. And that means trouble in two different environments worldwide. Just as the cyanobacteria are smothering the Great Lakes with their "blooms" and contributing to already polluted air, the "red tide" is fouling coastal areas and killing large numbers of fish and sea mammals. Florida has been hit especially hard this year, as you can see from this recent photograph. (Not all of these blooms are red, nor are all of them made by dinoflagellates.)

Dinoflagellate (field drawing)

Dinoflagellates are many kinds of one-celled protists that come in a multitude of diverse forms.  Actually, there are probably more dinoflagellates than there are other kinds of protists.  Protists are neither plants nor animals, but may display characteristics of both. Some are as large as 2 millimeters in diameter, meaning that they can be seen with the naked eye. The one shown here is capable of photosynthesis because eons ago they began to take in organisms like cyanobacteria that could make food by photosynthesis. Instead of consuming these organisms, they just kept them. They went on consuming other small organisms and digesting them. Over a long period of time, the "inclusions" became "organelles" — little almost-organs that provided a specific function for their host. (You can see many blue and brown ones as dots and little blobs inside the organism. The red area is the nucleus and is capable of responding to light.)  As it grows larger, the host divides into two identical organisms by splitting in half, each half taking its organelles with it.  Dinoflagellates got their name because they are propelled by two whip-like flagella. The ones on this species seem extremely short.

Bad as the "red tides" are, there is a much more pressing problem with dinoflagellates. Long ago some of them formed partnerships with corals. Bear in mind that they are protists, while corals are animals. A partnership between the two might last for thousands of years. Then conditions in the environment changed and the dinoflagellates "were not happy". They pulled out of the corals, turned into small hard bodies called "cysts" and sank to the bottom. We don't know how long they can stay as cysts — maybe indefinitely. The corals, on the other hand, weaken, bleach, and die — something that is happening all around the globe today. When the situation improves, the cyst can produce another dinoflagellate, which will find another kind of coral and start the whole cycle over again.

Mature, healthy coral reef

 

Jellyfish (field sketch)

Finally, we come to our "catch of the day".  The red organisms that fled from our lights and showed some bioluminescence. These were jellyfish, animals belonging to the same group as sea anemones and corals. They are probably the first animals to actually swim, rather than just drifting with the current. They are deceptively simple, little "umbrellas"with an outer layer of cells covered with a net of nerves, an inner layer that handles digestion, and muscle tissue between the two which enables them to swim. Their long tentacles have stinging cells that stun or kill prey. They often have light-sensitive cells around the edge of the "umbrella" that tell them whether it is day or night, the angle of the light, and how bright it is. Dimming of the light indicates other organisms and gives some indication of whether they are small enough to be eaten or big enough to be predators.

Box jellyfish eyes

Today, the deadly box jellyfish, named for their shape, live in shallow waters where there are obstacles like mangrove roots. They have 24 "eyes" arranged in clusters around the bottom of their bodies. There are 4 different kinds of eyes. These are arranged in sets of 6 organs. One kind is very simple and probably just separates light from dark. Another is more complex and possibly can detect different degrees of light intensity and direction and may give some idea of shapes.  But each cluster has a pair of organs that each have a cornea, a lens, an iris, and a retina. In other words, they are eyes (without the quotation marks)! This little animal does not have a brain, but it has functional eyes that allow it to form images, avoid obstacles, change its swimming speed, and always return to its "home base". It can even see above the surface of the water and may actually hunt prey, rather than just waiting for it to swim past. It appears that "simple" animals are not so simple after all. It also may be the earliest animal to sleep at night. I guess it needs to rest its eyes.

And that, my fellow time-travelers, is how Light from the Sun penetrated Earth's waters and, bit-by-bit, the "simple" organisms developed the power of Sight.

Have fun. Stay safe.

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