Looking past old bones

 kw: book reviews, nonfiction, science, geology, paleontology, plants, evolution

Cyanobacteria, which were called "blue-green algae" when I was taking Freshman botany more than sixty years ago, arose about 2.7 billion years ago. The Earth started to become green. Before that it was primarily orange. The greening of Earth began in earnest once certain cyanobacteria became incorporated into the cells of an Archaean species to become chloroplasts in the first eukaryotic cells. Multicellularity, in the sense that among a bunch of cells that were sticking together, the cells began to have different functions, came about perhaps 1.5 billion years ago. By 1.2 billion years ago a proto-alga we call Bangiomorpha was the tallest plant in the shallows of the Precambrian ocean, being all of 2 mm tall, towering over micron-sized prokaryotes.

Bangiomorpha features in the first chapter of When the Earth was Green: Plants, Animals, and Evolution's Greatest Romance by geologist Riley Black. The chapter is titled "Sex in the Shallows" because it is pretty certain that Bangiomorpha was among the first organisms to reproduce via sexual differentiation of gametes and gamete fusion.

Side note, not related to the content of the book: The preference in nature to use DNA rather than RNA for long-term storage and retrieval of genetic information is based on its stability. RNA copying is much more prone to error, and the lack of pairing of RNA strands makes an "RNA world" very fragile. However, RNA is essential for helping DNA make copies of itself, and for translating sections of DNA into proteins. The fact that the biosphere was extremely slow to change during the first billion years after life began indicates that DNA is "too slow"! The mix-and-match processes of meiosis and gametogenesis sped things up appropriately, so that complex life could arise before the Sun burned out (not that "nature" had any idea it had such a deadline). Sex combines a level of stability much greater than an RNA basis allows, while also providing mechanisms for making changes more rapidly, particularly when small populations are isolated under adverse conditions. As you might imagine, biological theorists are still arguing strenuously about the pace of evolutionary change at various levels.

Back to the book: The fifteen chapters each tell a story of a particular organism or ecosystem, focusing on the plants. Most of paleontology is "shells and bones". Having done my time clambering over desert landscapes all over North America—as Riley Black is still doing, lucky kid!—I relate strongly to the tendency to focus on the hard bits. It is easy to climb a cliff that was a marine reef in the Pennsylvanian Era and snatch up brachiopods, bryozoans, clams, snails, and corals. The plants that accompanied them are simply not in evidence; they don't fossilize well in the gritty sands that surround the structures of the reef. And why collect fossils in deserts? I live in Delaware now, and most rocks containing fossils are buried under tens of feet of soil. In Nevada, there are lots of rocks right at the surface.

Some of the chapters tell stories that feature sundry animals, retaining the emphasis on the plants they lived among, and often upon. For example, Chapter 13, "Far from the Tree" has a proto-hyena watching two monkeys squabble on the branch of a tree in late Miocene Ethiopia, hoping one will fall. The tree is the "hero" of the story, along with the effects tree dwelling had on the morphology of the little primates. The focus of the chapter is the gradual spread of grasses as the forests retreat. 

By the way, it is stated that grasses have C3 photosynthesis, making them more efficient than trees at turning CO2 and water into carbohydrates. This is an error. Grasses are the primary C4 plants, while most shrubs and trees use the older C3 photosystem. I don't know if this is a typo; it should have been caught by the copy editor at very least (The author had a copy editor, who is named in the Acknowledgements). Anyway, to my point: Prior to the evolution of C4 photosynthesis, CO2 content of the atmosphere was nearly always between 500 ppm and 2,000 ppm. C3 plants thrive best with at least 500 ppm. C4 plants can draw down CO2 below 100 ppm, at which point most trees won't grow at all. So, you folks out there that want to grow stuff to draw down CO2, use grasses, not trees. Even at today's CO2 level of 440 ppm, the trees are struggling.

My favorite chapter is 3, "The Forest Primeval", in which the author lyrically describes aspects of life in the Carboniferous Era (as it is called in Europe; in North America this era is divided into the Mississippian and Pennsylvanian Eras).

This image I generated using Seedream V4.5 in OpenArt is not as densely packed with trees as I wanted, but it shows the alienness of the treelike species that existed at the time. Of course, I had to include an eagle-sized dragonfly. The rapid profusion of plants and their equally rapid burial, which formed our coal beds, led to a very high level of atmospheric oxygen. As the author points out, the larva of the dragonfly needed to be larger to reduce oxygen toxicity. Yet the extra oxygen also fueled the energy needed for the adult to thrive at its meter-sized length.

Had I been the publisher's editor, I might have amended the book's title to just The Earth was Green. This image of the Blue Ridge in Tennessee, from a panorama by the National Park Service, shows that Earth is very green even today:

Riley Black is a trans woman, who put the pronouns "she/they" in the author bio. Since she is at least halfway through a full physical transition, I'm willing to say "she". "They"? Not so much. I care about singular and plural. Her pronoun confusion extends to the grammar of the book, where it is applied to the creatures. Sentences with wording such as, "Now that the monkey can reach the fruit, they can…" are simply solecisms. The common usage is "it can…", which is at least genderless. Similar grammatical errors are found numerous times per chapter. It is sad. Apparently her copy editor shares her grammatical mis-education.

Ms Black's writing is lyrical and enjoyable. She has published several earlier books, and I understand all are popular. And I envy her access to so many prime fossil localities!

One last quibble: In the first chapter, describing how DNA data are mutated, the analogy of a copy of a copy of a copy in a photocopier is used. It's a very bad analogy. The successive copies gradually fuzz into meaninglessness. No new letters appear. In DNA, every "word" is exactly three letters. Every possible combination is meaningful. A DNA copy error doesn't make any fuzz, it exchanges one letter for another, or it may even add or delete a letter, causing a frame shift (frame shifts are fatal flaws).