Which Came First: Eyes or Flower Colors?

By John Richard Schrock

Go back in time to well before the dinosaurs. What would the plant world look like? Plants most certainly would sport their green stems and primitive leaves because photosynthesis requires the absorption of red and blue colors, but reflects green. But where are all of the beautifully colored flower petals? The scene would likely be very drab and dull, absent the seasonal turnover of brilliant and beautiful flowers that light up our life each year.

This chicken-or-egg question provides the rationale for science writer Steve Parker’s placing the evolution of the eyes in front of the chapters discussing the evolution of our world of color. A Senior Scientific Fellow of the Zoological Society of London, Parker brings excellent high-quality photos to nearly every set of facing pages in this well-bound book published in Canada.

Over 90 percent of all animal species have structures that serve as eyes, from barely distinguishing light from dark, to visual acuity exceeding ours in wavelength and detail. While this book deserves shelf space in a biology classroom, it is written with enough clarity, and with illustrations that define the necessary details, to be read by any non-scientist. When the interest generated is great enough, nearly all of us can read above our assigned “grade level.” Here we find that long before critters crawled onto land, a box jellyfish with the most minimal of nervous system, possesses a two-pigment-cup eye. And take away the need to see, as in the case of blind cave salamanders, and the eyes devolve and disappear.

The Cambrian period 541–485 million years ago saw a burst of eye forms. And this book is up-to-date by his inclusion of recently discovered PAX genes, first evolved to control and regulate body parts but then co-opted to build eyes. And we now know that mutated PAX genes result in eye malformations. Trilobites were especially prevalent and their arrays of primitive lenses are only known from fossils today. Rods are the first photoreceptors, detecting wavelengths without distinction. Cone cells that have pigments responding to red, green or blue, appear to arise 359-299 million years ago in spiny sharks.

While the basic molecular design for detecting light probably just arose once, the many major changes in design are estimated to have arisen 50 to 100 times. The human eye is a case of a simple eye, the visual unit of mammals. This is contrasted with the compound eyes of today’s insects, where thousands of units fan out to receive a wide range of input. And an advantage of seeing depth arises with two separate eyes focused on the same view. Ciliary muscles can then change the lens to focus on detail. A predator uses forward focused eyes. Prey use eyes to the side of the head to alert them to predators. Variation also occurs in the wavelengths of light, with bees and some other critters seeing light wavelengths beyond our ability to perceive. And some snakes perceive size and shape by their perception of warmth. But dogs generally only perceive yellow from blue.

Thus we realize that colors in plant petals in part evolved to attract pollinators. Young insect collectors often catch the big green luna moth only to see it fade to white in their collection. Other butterflies retain their color for centuries. This is the distinction made between pigments and structural colors.

In the 1980s, I was in charge of sending out publication brochures to museums. I conducted a small experiment and printed them in four different color combinations: red-black, blue-dark blue, green-dark green and yellow-black. Equal numbers of each were sent around the country, but the orders that came back on the yellow-black brochure equaled all the other three combined. Yellow-and-black are known as warning colorations. I suspect this attention-mechanism resides to some extent in our genetic inheritance. Parker explains the Mullerian and Batesian mechanisms, where eyes and colors walk in step. So if this book review could be printed in black-on-yellow, even more folks would read it.

– John Richard Schrock is editor of the Kansas School Naturalist at Emporia State University in the United States and also teaches various classes at universities in China.