Looking Down the Tree – Exploring the Origins of Our Species: book report
We got news of Mitch Cruzan’s new book on human origins and invited him to write a short introduction, which you may see here. In the meantime, the publisher graciously sent a review copy, and I report on the book below.
Mitch Cruzan is Professor of Biology at Portland State University and has by his reckoning been teaching evolutionary biology for 15 years. I recently read an advance copy of his new book, Looking down the Tree – The Evolutionary Biology of Human Origins. Although I suppose I am a fellow traveler, I am not a biologist, and occasionally something went over my head. I wondered, therefore, who the book’s intended audience was. Not unlike a paleontologist, I searched within the book and immediately found on page 15, "You probably know about DNA from your biology courses." I frankly thought that he should have aimed higher: it would not have taken much additional explanation to prepare a more popular book and not what I think amounts to a supplementary college text. Indeed, I thought that it would have been useful to have defined terms such as apocrine gland, homologous, and recombination for readers who have never had a biology course.
Cruzan argues convincingly that to understand our phenotypes, you have to look at our past, not our present. He thus begins each chapter with an anecdote from the daily life of a fictitious hominid he names Launua. Launua belongs to a clan that is barely surviving, having comparatively recently ventured into the savanna. Her clan is inbred (presumably not so much that it is deleterious) and held together by kin selection, or inclusive fitness. Cruzan sometimes seems to conflate inbreeding with kin selection and remarks that inbred populations display kin selection. Each chapter ends with a useful summary.
Cruzan asks, for example, why we have opposable thumbs. Not, as I understand it, because we need them for grasping, but initially because our upright posture necessitated an elongation of our big toes (not all our toes), and our hands followed along because they use the same regulatory genes. Here he uses the large thumb to introduce the concept of an exaptation, though later he uses the obsolete but more descriptive term preadaptation. Cruzan thinks,
The single most influential event in the history of our lineage was the transition to savannas and selection for bipedalism, which caused correlated changes in the hands of our ancestors.
Cruzan argues that we lost our hair in favor of evaporative cooling. He argues further that we have hair on our heads for thermal regulation, which made me wonder why males (European males, anyway) often have heavy beards, not to mention more body hair than females. He does not discuss other animals that live on the savanna and are covered with hair. He uses head and body lice, which live in our hair, to cleverly deduce that we probably wore clothing made of woven fabric about 100,000 years ago.
A colleague of mine once remarked that if you want to get students’ attention, just say "testicle." Cruzan tests this hypothesis in spades and explains in some detail why human males have large penises compared to other apes, and human females have correspondingly long vaginas. (Spoiler alert: it is the other way around and comes about because of our large heads, not sexual selection.) I was not completely convinced by his claim that the human female breasts and buttocks are primarily for storing fat; anyone who has ever seen a picture of a baboon in estrus might propose a different hypothesis.
The grandmother hypothesis postulates that menopause evolved so that grandmothers would be available to help raise their grandchildren. In discussing this hypothesis, Cruzan claims that human females ovulate about once a month and thus run out of eggs. That seemed to me to be an anachronism; in nature, before contraception, they spent most of their adult lives pregnant or lactating, and possibly ovulated no more than a dozen or two times in their entire lives. Cruzan goes on to note, interestingly, that exclusive homosexuality is almost unique to humans and suggests that it is subject to selection even though homosexual men do not reproduce. Rather, he thinks they were helpers, similarly to the grandmothers. The argument is slightly abstruse and depends on the fact that humans have a constant sex drive, as opposed to the periodic sex drive in other animals.
The final chapter is somewhat depressing, in a way. Cruzan observes that "many millions—probably billions—of individuals had to die [we all die; I think he means die prematurely] as small improvements accumulated generation after generation over the past four to six million years." He goes on to say that today we "buffer" ourselves against natural selection by extending the lives of those who would otherwise not survive, and a decline in fitness is inevitable: "[D]eleterious mutations will continue to accumulate in populations with access to modern medicine," and these mutations will become a challenge. Though he does not quite commit himself, he points out, "[S]ome evolutionary biologists have warned that we can expect declines in health and longevity in future generations."
I suspect that the file I got was something along the lines of a galley proof, but still I did not find a lot of errors. The figures did not all stand on their own, and I thought that nearly every one would have profited from a detailed caption, as well as more-specific discussions in the text. Phrases such as "You may remember from a previous chapter" would have been more useful had they cited the actual chapter. "Gene genealogy" seems very awkward to me and probably should have been "genetic genealogy"; Google’s search engine agreed by a score of 430,000 to 29,000. The text alternated between "descendent" and the more common "descendant"; I hope that problem was fixed before the book went to press.
Want to know more? Get the book, read it – and, of course, comment below.
Acknowledgement. Andrew Petto, Senior Lecturer Emeritus at the University of Wisconsin–Milwaukee, lavished many words commenting on this essay (though he did not read the book) and explained several topics that had been unclear to me. Any errors are my own or may be blamed on my speech-recognition program.