Author: clangle1

Much Depends on Dinner

This chapter deals with how the Australopiths partially weaned us off fruit.  Chimps eat enormous quantities of fruit, and sometimes resort to lower quality food such as leaves and stems.  Lieberman argues that the second major transformation in the history of the human body is the trend toward dietary diversity.  The Australopiths lived between 4-1 million years ago and were obligate bipeds.  However, they still had ape-like traits such as small brains, long snouts, and browridges.  One difference from apes was that their diet had shifted from fruit to tubers, seeds, and plant stems.  The evidence for this are the large teeth and jaw, forward placed cheekbones, and large chewing muscles.  In fact, when the first Australopith was discovered, he was nicknamed “Nutcracker Man” because of his extremely large jaw.  Lieberman argues that the ancestors of Australopiths were forced to eat lower quality food because of the gradual cooling of the planet, which caused Africa to dry out.   These foods are termed “fallback foods” and are only eaten when necessary.  Evolution favors those who are able to eat fallback foods when normal foods are unavailable, because it is often a matter of life or death.  Lieberman quips that evolution logic dictates that sometimes, “you are what you’d rather not eat”.  Lieberman also explains that there is some evidence that Australopiths started digging for food for roots, tubers, and bulbs, which collectively are called underground storage organs (USOs).  Lieberman suggests that USOs became an important source of calories for Australopiths, because they are more starchy and energy rich than many fruits.  However, USOs are extremely fibrous and tough, meaning that they were very difficult to chew.  Lieberman theorized that Australopiths would eat them by wadging – which means chewing for a long time to extract nutrients and then spitting out the pulp.  This is reflected in the molars of Australopiths, which were about the size of a thumbnail, two times the size of chimpanzee molars.  Lieberman argues that this adaptation is still present in humans today, because our molars are still larger than chimpanzee molars.  Additionally, foraging for USOs requires more walking, so efficient walking must have been very important.  Although bipedalism prevented Australopiths from galloping, bipedalism allowed for efficient walking and freed the hands from digging or carrying food.  Lieberman argues that Australopiths were a key intermediate stage in human evolution, and without the adaptations that they developed, we would not be here today.

More information about Australopiths!

Understanding Apes: How we Became Bipeds

This chapter starts with a question:  How did humans become so ill adapted to life in the trees, as well as feeble, slow, and awkward?  The catalyst for this was likely bipedalism, when our ancestors left the trees to walk on two feet.  The split between chimps (our closest relative) and humans is estimated to have occurred 8-5 million years ago, but no fossils of the last common ancestor of chimps and humans has ever been found.  Lieberman lists proposed early hominins, such as Sahelanthropus, Orrorin, and Ardipithecus ramidus.  One common trait between all of these fossils is the ape-like features, including small brains, browridges, big front teeth, and a protruding snout.  Lieberman argues that these organisms were all hominins because the defining trait of hominins is adaptations to walking upright.  Lieberman also makes the point that occasional bipedalism is not uncommon, in fact, apes and many other mammals can walk on two legs. Only the habitual bipedalism of humans is unusual.  He explains the important traits that allow humans to walk upright efficiently, including orientation of the hips, S-shaped spine, and the arch of the foot.  Early hominins acquired these traits while also retaining some distinctly ape-like traits such as opposable big thumbs, curved toes, and slightly tilted ankles.  Lieberman makes the argument that these first hominins were occasional bipeds, who were able to walk efficiently, but also able to climb trees.  However, because adaptations cause tradeoffs, they were likely less efficient at climbing trees than chimps, and less efficient at walking than humans.  Now the question is why did bipedalism evolve?  Lieberman suggests that climate change was the trigger for the evolution of bipedalism.  As the climate got colder, Africa started drying out and our ancestors may have had to travel further to find food.  Lieberman has found that apes spend four times more energy than humans do while walking, which shows that bipedalism may be a more efficient way to walk long distances.  Lieberman does acknowledge that more research in this area is needed to find more evidence supporting this hypothesis.  Either way, bipedalism was a key change that allowed for more changes, such as tool making, but it also caused major problems.  A couple of problems include back pain during pregnancy, loss of speed and agility, and loss of the ability to climb trees.  But somehow, the advantages of bipedalism outweighed the disadvantages.

Introduction: What Are Humans Adapted For?

The introduction follows the escape of the “Mystery Monkey” from a zoo in Tampa, Florida in 2012.  The monkey had evaded capture for three years until he was captured in October 2012.  People marveled at the fact that a monkey was able to survive for so long in a city, a place where monkeys clearly do not belong.  However, Lieberman makes the point that humans are just as out of place in cities as a monkey.  Humans evolved as hunter-gatherers, and lived in groups of no more than 50.  This leads to the question, what exactly are humans adapted for?  And what does this mean for how humans should live today?  Some have attempted to live a more Paleolithic lifestyle, by avoiding processed foods and exercising regularly.  However, Lieberman makes the point that just because we evolved to eat certain things doesn’t mean they are good for us, or that there aren’t better options.  Additionally, not every trait is considered an adaptation, because not every trait is advantageous.  It’s important not to assume that particular traits are advantageous without evidence, even if the idea is intuitive.  Adaptations are driven by environmental factors, and always involve compromise.  Lieberman gives the example that for humans short legs are advantageous for conserving heat in colder climates, but makes walking and running less efficient.  It is impossible to evolve into a “perfect” organism because of the nature of adaptations.  Most importantly, humans did not evolve to have long, healthy lives.  Instead, humans evolved to live long and healthy lives, but only if this produced more surviving offspring.  Lieberman answers the question of “What are humans adapted for?” with a short answer of to have as many children as possible.  But in reality, humans are not adapted for any specific diet, habitat, environment, or exercise, and there is no such thing as optimal health.

However, one consequence of our evolutionary hypothesis is the mismatch hypothesis.  Adaptations that were once advantageous, such as craving energy rich foods and efficiently storing calories as fat, are now detrimental to our health in modern society.  When these adaptations work against us, it results in chronic diseases, such as diabetes.  The mismatch hypothesis is the focus of the second half of the book.  However, to fully understand the reasons for modern chronic illness, one must understand what humans are adapted for, which is convoluted and messy.  Additionally, we must understand both biological and cultural evolution.  The main cultural changes include the Agricultural Revolution and the Industrial Revolution, both of which had profound effects on the environment that humans lived in.  If we gain a better understanding of our adaptations and the effect of cultural evolution, we can gain a better understanding of what is causing mismatch diseases.  Unlike genetic disorders, which are passed on by biological evolution, mismatch diseases are passed on by cultural evolution, where we pass on the same environment and habits to our children, creating a feedback loop.  This feedback loop can only be broken by knowledge of the factors that cause mismatch diseases in the first place.