The story of human evolution is constantly being refined with new findings and there is a glut of accessible books that cover this topic from various angles. Yet, with The Cradle of Humanity, geography professor Mark Maslin manages to provide an interesting and novel take on the subject, showing the reader how a happy combination of larger factors conspired to influence and steer our evolutionary trajectory. It could have ended up so differently…
From the above, it is immediately clear that Maslin subscribes to Stephen Jay Gould’s idea of contingency laid out in Wonderful Life: The Burgess Shale and the Nature of History. To the question posed by a book such as Improbable Destinies: How Predictable is Evolution?, Maslin’s answer is “not very”. If you could rewind and replay the proverbial tape of life, you would not get the same outcome.
Maslin starts off with an excellent primer on early human evolution that quickly outlines all the currently known hominin genera, such as Paranthropus, Australopithecus, and, of course, Homo. New fossil finds constantly update and refine that story, so you can expect that a book like Our Human Story: Where We Come from and How we Evolved (an updated version of its 2007 predecessor) will need to be updated again in the future. Elsewhere I have already discussed the insights that we are gaining from the analysis of ancient DNA (see my review of the highly recommended Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past). It is pleasing to see Neanderthals making a return later in the book. Other than the fact that we crossbred with them (see Neanderthal Man: In Search of Lost Genomes), their story is being rewritten by new findings. Far from primitive brutes, these close cousins of ours used tools, buried their dead, adorned themselves – in short, were far more sophisticated beings than we used to give them credit for (see The Neanderthals Rediscovered: How Modern Science is Rewriting Their Story and my review of The Smart Neanderthal: Cave Art, Bird Catching, and the Cognitive Revolution for more).
“It is pleasing to see Neanderthals making a return later in the book […]. Far from primitive brutes, these close cousins […] were far more sophisticated beings than we used to give them credit for”
After this warm-up, Maslin launches into his main argument. What were these large factors that influenced our evolution? Obviously, you need a habitable planet with just the right conditions. Rather than retread the general overview put forward in books such as How to Build a Habitable Planet: The Story of Earth from the Big Bang to Humankind and The Goldilocks Planet: The Four Billion Year Story of Earth’s Climate, Maslin effectively says “let’s say take that starting point for granted”, and zooms in on East Africa and the last few million years. Why East Africa? Because this is where many fossils are found and our story as humans seems to have started.
In short, well-illustrated chapters Maslin discusses the influence of astronomy and geology on the planet’s climate. First, there is the combination of several wobbles in the planet’s orbit and axis of rotation (technically speaking eccentricity, obliquity, and precession, but Maslin does a far better job than I do here explaining them). Each of these has their own periodicity and they can neutralise or amplify each other, affecting how much sunlight the planet, or parts of it, receive. This planetary pulse (technically known as Milankovitch cycles) is the driving factor behind the cyclical pattern of ice ages over the last few million years.
Maslin also introduces the geological history of the East Africa Rift System, which has formed at the boundary of several continental plates meeting in East Africa. Geology, specifically plate tectonics, influences ocean circulation and weather patterns. Elsewhere (see my reviews of The Oceans: A Deep History and Brave New Arctic: The Untold Story of the Melting North) I have already highlighted that the coming together of various mechanisms results in fiendishly complex climate patterns that are very hard to predict and sometimes have counterintuitive effects. Maslin, however, excels at explaining phenomena such as monsoons, the El-Niño Southern Oscillation and the influence of land bridges and mountain building episodes.
“Why such a large brain? It is energetically expensive and makes childbirth (quite literally) a pain.”
The theory that Maslin and others have developed is that pulses of climate variability interrupting longer periods of stability were the driving force behind the evolution of new hominin species. It is an attractive model, though it is not the only one out there and Maslin is perfectly at ease arguing that his and other models might all be equally valid explanations.
There are two other topics that Maslin shortly tackles in the last two chapters. The first is our large brain. Why? Why such a large brain? It is energetically expensive and makes childbirth (quite literally) a pain. Here, too, there are many explanations, but Maslin sides with the camp that argues that larger brains helped us navigate living in large social groups. Language is, of course, an important part in this, and he shortly explores the various ideas that have been floated to explain its evolution. Similarly interesting is the idea of self-domestication: changing skull morphology during our evolution – away from heavy-browed skulls to lighter and flatter skulls – suggests a drop in testosterone levels, which would result in less violent behaviour and more social tolerance (see also my review of The Goodness Paradox: How Evolution Made Us Both More and Less Violent).
“[The book features] 50 helpful illustrations and diagrams […] something I rate very highly in a book”
The other topic is the Anthropocene, on which Maslin has written more extensively in The Human Planet: How We Created the Anthropocene. By now, you will no doubt have come across the idea that humanity is altering the planet at such a scale that ours can be considered a new geological epoch (see The Anthropocene as a Geological Time Unit). Maslin very briefly surveys our impact on climate (see also Plows, Plagues, Petroleum: How Humans Took Control of Climate), the biosphere (see also Harvesting the Biosphere: What We Have Taken from Nature), and biodiversity (see also The Sixth Extinction: An Unnatural History).
In a book of this brevity, topics are necessarily treated in a cursory fashion only, and each section could function as a springboard from which to explore further, as I have tried to show throughout this review. Luckily, I was taken by Maslin’s style. I can’t quite put my finger on it, but it is very to the point, wasting little space on fluff. His clear explanations are combined with over 50 helpful illustrations and diagrams that clarify geological, climatological, and evolutionary facts and processes. This is something I rate very highly in a book, and it is something many publishers, unfortunately, pay too little attention to in my opinion. The result is a superb and highly recommended book that convincingly argues how the happenstance conditions in East Africa shaped us and our forebears.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
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