Evotour, part two. The Southwest Desert

Immediately after seeing the eclipse of the Sun, I headed to Petrified Forest National Park. Even though it was over 100 degrees, I still found it utterly captivating. This is a place that offers a wonderful way of looking into the evolutionary past.

The Petrified Forest today is desert, with only the skimpiest cover of shrubs and grasses, but over 200 million years ago, during the Triassic Period, it was a flooded forest dominated by Araucarioxylon arizonicum trees. The climate today is entirely different from what it was 200 million years ago. Entire petrified trunks are strewn over the ground, having eroded from the ancient sediments. Under wet, anaerobic conditions, the trunks decomposed slowly enough that silicate minerals diffused in and replaced the organic molecules, with the result that the trunks, which are now rocks, retain much of the original structure, including easily-visible wood grain. In the cross-sectional fractures, the ring structure of the wood can still be seen, though not in enough detail to study individual rings. Along with the silicon, minerals such as copper and iron have created astonishing colors. This is one of the best places to observe the effects of the processes of fossilization.

The presence of these fossilized tree trunks provides strong evidence for the long evolutionary history of the world. Creationists claim that petrified wood can be produced rapidly, and that the Petrified Forest could have been produced during a single Flood of Noah. They claim that this is just a Flood deposit. But all of the plant fossils in the Petrified Forest vicinity represent extinct forms of conifers, not just Araucarioxylon but also Woodworthia arizonica and Schilderia adamanica. Smaller plant fossils include clubmosses, ferns, cycads, and ginkgoes. Note that there are no modern conifers (such as pines) and no hardwood trees or flowers. How could Noah’s Flood waters have selectively picked out just conifers, clubmosses, ferns, cycads, and ginkgoes, and excluded all the hardwoods and flowering plants? The only reasonable explanation (without invoking a special miracle of which the Bible gives no hint) is that this deposit comes from a time in Earth history when hardwoods and flowers had not yet evolved.

There is a similar pattern in the animal fossils. Vertebrate fossils include crocodile-like phytosaurs, large Buettneria amphibians, and early dinosaurs. These deposits come from a time in Earth history when larger dinosaurs, mammals, and birds had not yet evolved. A Flood could not have picked out just the amphibians and small dinosaurs, and excluded all the large dinosaurs, mammals, and birds.

The next day I visited the Sonoran Desert. Most of this desert is behind fences and inaccessible from highways. But I managed to find a Maricopa County park south of Phoenix in which I could walk among the saguaros and other desert plants. Even though it was so hot that my video camera flashed a heat warning, I was utterly fascinated by this location. It is a showcase of evolutionary creativity. All of the plants were adapted to hot, dry desert conditions, but each of them in different ways. Most noticeable are the saguaros, which have spines (that keep animals from consuming their tissues to get water) and have a special kind of photosynthesis in which they store acid during the cool night and keep their pores closed during the day, when they manufacture sugar (this is known as CAM photosynthesis). But there are also the palo verde bushes. Palo verde means green stem, and these small trees have green stems rather than green leaves. There were also creosote bushes, which had small leaves, but the leaves gave off an unmistakable creosote scent. The volatile chemicals that create the scent actually provide heat stabilization to photosynthesis. Finally, there were black crusts on the soil surface, which are the resting phase of microscopic algae that come to life during rains and only during rains. The spring ephemeral wildflowers, which have adapted to the desert by growing like crazy for just a few brief weeks after the winter rains, were already gone. Evolutionary adaptations take many different forms, precisely because each group of organisms finds its own path of adaptation.

As a matter of fact, CAM photosynthesis has evolved many times, in different families of succulent plants. This is another fascinating evolutionary story. The enzymes involved in CAM did not evolve from scratch; they already existed and were used by these plants’ ancestors for a different function. Evolution borrowed them and repurposed them for this special kind of photosynthesis, and did so several different times in different parts of the world.

I apologize for the fact that two of the photographic images are on their sides, but Blogger insists on orienting them in that manner and there is no way I can change it. I saved the files in vertical orientation. Guess that's what you get for having a free blog from Google. I hope to post YouTube videos about the eclipse and the desert soon.

Maybe I stayed a little too long out in the Petrified Forest and the Sonoran Desert, because after I arrived in La Jolla to visit my sister, I experienced the temporary symptoms of recovery from heat prostration. But it was a fascinating exploration of evolution, one that I hope you will be able to experience someday yourselves.

Note: Someone posted a comment on a previous entry, but when I click on it, my computer goes into an unresponsive mode. I have not read the comment but I suspect that someone has inserted a virus. Beware!  Update: The problem was apparently a computer glitch caused by Google. The comment was not a virus.