Looking Down the Tree – Exploring the Origins of Our Species
Mitch Cruzan is Professor of Biology at Portland State University. He received his BA and MA in Biology from California State University, Fullerton, and his PhD from Stony Brook University. He is currently an associate editor for Molecular Ecology, a leading journal in the field. He has published more than 80 papers in the primary literature and an advanced textbook, Evolutionary Biology: A Plant Perspective. His latest book, Looking Down the Tree – Exploring the Origins of Our Species, is due to be published in a few days, and we have invited him to offer a preview here.
I have offered an introductory course on evolution annually over the last 15 years. I always saved in-depth discussions of human evolution to the end for two reasons. First, to discuss human evolution in a meaningful way, we need to have a good understanding of evolutionary principles and processes. Second, I thought it was a good highlight for the course to end on. Each year, I incorporated new information and new ideas about human origins. Much of this content came from the questions that students asked; they weren’t satisfied to look a series of fossil remains or boring details about human genomics. They wanted to know more about the origins of our unique appearance and behaviors, and particularly secondary sexual characteristics.
Why does anyone take on a particular subject for a non-fiction book? You might ask, what could a plant evolutionary biologist such as myself possibly have to say about human evolution? Some reviewers actually said that I had no business writing this book. But evolutionary biology consists of a set of principles and processes that can be applied to any organism. Yes, it helps to have knowledge of the biology of a species, but for humans, that information is readily available. For me, developing the material was a natural process as I explored aspects of human evolution, along with the students in my introductory evolution lectures. By the time the pandemic hit, I had developed what I thought was a unique story of the origin of human traits – one that made sense in the context of the habitats our ancestors inhabited.
This book is not just a rehash of previous statements and ideas. By integrating information from disparate fields such as paleontology, anthropology, and genomics, I have drawn unique conclusions about the origins of human traits and behaviors.
Reading previous books and sources discussing human evolution on the web was frustrating because of how much misinformation was available. It was also frustrating that many inferences were made from observations of humans in modern societies, which is irrelevant to the origin of human traits. I decided to do something to counteract the wide range of – sometimes crazy – ideas. But I did not want to present information in a dry, dictatorial monograph. I thought that it was important to share what I had learned with as broad an audience as possible and to make it engaging enough to hold the attention of someone with little background in biology. To accomplish this, I made sure to keep things simple while being as accurate as possible. It was a tricky balance, but I managed to keep the writing at the high school level.
I also strived to provide adequate levels of detail without including too much tangential information, so the end product is relatively short. In addition, I use a conversational writing style to lead the reader through the evidence with logical arguments so they will understand how we come to conclusions – how we do science. To vividly illustrate the challenges and struggles our ancestors experienced, I include a fictional story of someone who lived around 70,000 years ago. I use vignettes from her life to introduce the content of each chapter and to exemplify the evolutionary principles discussed. I expect not all readers will appreciate this approach, but those who don’t can just skip the vignettes without missing the majority of content.
I hope that readers will find my discussions of the origins of unique human traits thought provoking and enlightening. I was not involved in collecting the data that I cite, but realize that sometimes you need to step back to see the connections that are not obvious from the perspective of individual research silos. By the end of the book, I hope readers will come to understand the origins of human traits and connections among them. Like those of any other animal, our unique appearance and behaviors are products of natural selection as our ancestors struggled to survive in the harsh and challenging environments they inhabited.