Category Archives: Evolution

Ask the Biological Anthropologist: Issue #2

Last year I introduced a Q&A feature to the blog, inviting any and all questions related to primates/evolution/anthropology.    Today I’m pleased to present Issue #2 of Ask the Biological Anthropologist!

Q: Since evolution relies on the mixing of genes to create new offspring, with the chance that a random mutation can result in something “interesting,” do organisms with longer replication intervals fare poorer at the evolution game than those with very short replication intervals (like viruses)?  –Matt, Arlington

A: This is a great question.  But I’m going to pause to clarify some terminology and basic reproductive biology before I give an answer that will complicate the dichotomy Matt has proposed.

A Note on Terminology: As I explained way back when I first started this blog, it’s important to distinguish between ‘evolution’ and ‘natural selection.’  Evolution describes the gradual changes that occur in populations as new genetic/physical/behavioral traits emerge.  These changes, however, are driven by natural selection, which is a process that acts on individuals.  Essentially, individuals must compete for the resources necessary to survive and reproduce, and those individuals who have more advantageous traits (i.e. those that are the most ‘fit’ in a particular environment), are able to reproduce more and pass on these traits.  I emphasize this because while, as Matt’s question suggests, mutation is the ultimate engine of evolutionary change, beneficial mutations don’t just produce something “interesting.”  They produce adaptations that improve an individual’s odds for survival/reproduction, leading those advantageous traits to become more common in subsequent generations.

A Note on Reproductive Biology: Mixing genes from two parents is not the only way to create offspring!  Asexual reproduction, in which a single organism reproduces by passing on its full complement of genes (producing, in theory at least, a genetically identical individual), occurs in species such as bacteria, sea stars, and many fungi and algae.  Viral replication is a more complex process, but still different from sexual reproduction, in which gametes from two parents (i.e. egg and sperm) combine to produce a genetically novel offspring.  As for mutations — changes to a DNA sequence due, for example, to copying errors during cell division — they can occur in any of these reproductive scenarios. More often than not mutations have deleterious or neutral effects but, as stated above, natural selection will favor a mutation in those cases in which it produces a trait that confers an advantage to survival/reproduction.

wolverine

Such as incredible healing powers that allow for the implantation of adamantium claws.

So how do mutations and a species’ method of reproduction relate to replication intervals — what I will refer to as generation times — and species-level competition?  Let’s start by thinking about those species that have short generation times.  In populations of such species, mutations arise more frequently simply because new individuals are being produced more often: individual members of these species reproduce at frequent intervals, and may  produce many offspring with each reproductive event.  On the one hand, this means that beneficial mutations can appear in these species more often, allowing populations to evolve more quickly as natural selection favors the rapid spread of new adaptive characteristics.  In the case of asexually reproducing species, however, there are two major downsides.  First, mutations are the only way to introduce new genetic variation into the population.  Second, deleterious mutations are also able to accumulate and, with no mechanism for ‘correction,’ may raise the likelihood of extinction (see: Muller’s Ratchet).

Species with long(er) generation times provide a notable contrast in both life history pattern and reproductive strategy.  “Life history” is a term used in biology to refer to the pacing or scheduling of events in an organism’s life, and encompasses factors such as rate of maturation, age and size at first reproduction, frequency of reproduction, size and number of offspring, and total lifespan.  Life history theory posits that for any species, all of these factors have been shaped by natural selection to maximize individual reproductive success in the context of specific ecological challenges (e.g. predation pressures).  The result is that species with a ‘slow’ life history reproduce more infrequently and produce relatively few offspring with each reproductive event.  Of particular interest to us in the present context, they also tend to reproduce sexually.

Romance is optional.

Romance is optional.

But why??  The truth is that sex has long been considered a bit of a conundrum from an evolutionary perspective.  It is costly, both in terms of the time and energy that individuals must use to find, access, and potentially keep a mate, and in terms of the fact that a sexually reproducing individual is able to pass on only 50% of his/her genetic material to each offspring.  From a fitness standpoint, this means that a sexually reproducing individual must produce twice as many offspring as an asexually reproducing individual in order to pass on its genes as successfully.  But as the differences in life history patterns mentioned above illustrate, this is highly unlikely.  So why?  Why rely on such a complex and costly system?

It turns out this isn't a great answer in evolutionary biology.

It turns out this isn’t a great answer in evolutionary biology.

This brings us back to species-level competition.  The Red Queen hypothesis, proposed by WD Hamilton, states that sexual reproduction is widespread, especially among species with long generation times, precisely because one of the perks of sex is that it produces offspring with increased genetic variability.  This is a necessary consequence of the mechanics of sexual reproduction —  the process of creating haploid gametes and having them fuse to combine the DNA/chromosomes of two individuals creates opportunities for what is known as genetic recombination — and is, according to Hamilton, a key tactic in the ‘arms race’ between parasites and host species.  Put another way, the reproductive mode of species with long generation times is likely an adaptation that compensates for the faster rates of reproduction and evolutionary change characteristic of parasitic organisms.

This is an important point, so I’m going to hammer home the logic:

  • Because they have short generation times, parasites undergo rapid evolutionary change that allows them to adapt to the most common host genotype.
  • This means that genetic diversity and new combinations of resistant genes — exactly the outcomes produced by sexual reproduction and genetic recombination — are important ‘counterstrategies’ in host species.
  • Sexual reproduction allows species with long generation times (such as humans) to ‘keep up with’ viruses and other potentially threatening organisms that have faster life histories.

Just as the Red Queen describes in a passage in Lewis Carroll’s “Through the Looking Glass”:

Illustration by John Tenniel, courtesy of Wikipedia

Well, in our country,” said Alice, still panting a little, “you’d generally get              to somewhere else — if you run very fast for a long time, as we’ve been doing.” “A slow sort of country!” said the Queen. “Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!”

This may seem like a very long-winded answer to a seemingly straightforward question about how reproductive rate affects success in “the evolution game.”  But hopefully I have made it clear that it’s not quite as simple as ‘fast’ vs. ‘slow’ reproduction, and that while natural selection may favor different life history patterns in different environments, it also produces adaptations that even the playing field in other ways.  The Red Queen hypothesis, for example, has been supported by data demonstrating that animals with longer generation times have higher levels of genetic recombination (Burt & Bell 1987).  So cool!

Natural Selection: Helping stack the deck in your favor for over 1 billion years.

Stay tuned next week for Issue #3 of Ask the Biological Anthropologist!


References Cited:
1. Burt A, Bell G (1987) Mammalian chiasma frequencies as a test of two theories of recombination.  Nature, 326, 803-805.

Further Reading:
1. Fabian D, Flatt T (2012) Life History Evolution.  Nature Education Knowledge.  [http://www.nature.com/scitable/knowledge/library/life-history-evolution-68245673]

Sgt. Macaque and Dr. Shepherd, reporting for duty!

What’s that?  You’ve never heard of Sgt. Macaque or Dr. Shepherd?  Well that’s probably because I just made those names up.

Dr. Dog

What do you mean “questionable credentials”?

I did so, however, based on two very real headlines that I happened across in the past few weeks.  Both are fascinating examples of humans taking advantage of other species’ adaptations to help us meet our goals, and both got me thinking about the prevalence and significance of this kind of human/animal ‘partnership.’

Headline #1: “Meet the Monkeys Keeping Chinese Troops Safe” 

For those of you not inclined to click on the link, I will summarize.  The People’s Liberation Army of China (aka the Chinese military) has revealed that a small unit of trained macaque monkeys is being used at one of the country’s Air Force bases to prevent migrating birds from nesting in the area and potentially getting sucked into aircraft engines (an occurrence that is good for neither the bird nor the aircraft/pilot).  According to the PLA, the monkeys, which are trained to destroy birds’ nests in response to whistle commands, are proving a more effective deterrent than scarecrows, netting, firecrackers, or human soldiers.  Not surprising, given that monkeys have evolved to be more adept at arboreal maneuvering than men of either flesh or straw.

And in other news…..

Headline #2: “Dogs Sniff Out Prostate Cancer With 98 Percent Accuracy”

This one is pretty self-explanatory.  Researchers in Italy trained two German Shepherds that had previously worked as explosive-sniffing dogs to recognize the scent of volatile organic compounds — chemicals associated with cancerous tumors — in urine samples.  In a subsequent blind study of almost 700 men, the dogs correctly identified which urine samples came from men with prostate tumors 98% of the time.  This success echoes previous research with medical detection dogs, which has found evidence that dogs can detect lung and breast cancers by smelling a person’s breath, and that they can be trained to warn individuals with diabetes or epilepsy of low blood sugar or impending seizures.

Considered alongside the other tasks in which dogs aid humans (some, such as search-and-rescue efforts and drug-sniffing, are well-known; others, such as identifying diseased beehives, sniffing out bedbugs, and tracking orca poop, are not), two inescapable conclusions appear:

1. Dogs have an amazingly good sense of smell.  Their noses contain up to 300 million olfactory receptors (ours have a measly 6 million), and a substantial portion of their brain is devoted to analyzing the smells registered by those receptors.   This means that they can smell in parts per trillion.  Imagine being able to smell one drop of blood in 20 Olympic swimming pools worth of water (sharks, by contrast, smell in parts per million or billion), and you’ll start to get an idea of how natural selection has honed this adaptation in canines.

2. Humans are good at co-opting it.

Best picture ever? Dog in homemade bee suit. Image credit Josh Kennett.

Humans, in fact, have created quite a niche for ourselves by exploiting other animals’  abilities.  And we’ve been doing it for a long time.  Recent evidence suggests that humans have been co-evolving with dogs, the first domesticated animal, for 30,000 years.  The domestication of farm and labor animals was more recent but, as evolutionary biologist Jared Diamond has thoroughly explained, it has had an enormous impact on the development of human societies over the past 10,000 years.  The truth of the matter is that humans just wouldn’t be where we are today if we didn’t have these conscripts.

But here, I think, is where it is worth making a big distinction between the subjects of the two headlines above.  Macaques are not domesticated.  Nor are a variety of other species that humans have put into service more recently, such as military marine mammals.  And this raises some major ethical dilemmas.  Is it okay for humans to use animals in this way?  For decades, the US Navy has taken advantage of dolphins’ swimming and echolocation abilities to detect and clear mines (good for the human population), but military sonar has simultaneously contributed to making the ocean a more unhealthy environment (bad for marine wildlife).  How much of a qualitative difference is there between this and the efforts of an organization like Helping Hands, which trains capuchin monkeys to act as service animals to the severely disabled?  Under what circumstances, if any, does human need trump an animal’s (or species’) right to live undisturbed in its natural habitat?  Do the “rules” differ for domesticated vs. non-domesticated species?  As someone who has admitted to having a Grand Canyon-sized soft spot for animals, these are questions that I genuinely don’t have answers for, and I am curious to hear other people’s thoughts.

In the meantime, I will finish with one more current news story, this time about a handful of humans doing something to help out animals:

Headline #3: “Wolf Pups Rescued From Funny River Fire in Alaska’s Kenai National Wildlife Refuge”

Image credit Kenai National Wildlife Refuge, via livescience.com

Thanks, guys.


Medical Detection Dogs and the In Situ Foundation are but two of myriad organizations devoted to training dogs to use their noses in service of human health.  Read this essay, however, for an alternative perspective on the use of dogs in medical detection.

New Q&A series

Last week I solicited friends and family to help me initiate a new feature for the blog: Ask the Biological Anthropologist!  I invited people to submit any and all queries related to primates and/or evolution, and I received so many great questions that I’ll be making ‘Ask’ a regular feature henceforth.

For this initial installment, I’m going to tackle a couple of ‘softballs.’  This isn’t to say they’re not great questions —  I actually really enjoy the opportunity to discuss things that may be common knowledge to primatologists but are completely new to people outside the field — but the answers are relatively straightforward.  So without further ado…..

Q: What are the differences between monkeys and gorillas, and how do you tell them apart?    -Gladys, Cambridge

A: I’m so glad someone asked this question because it is a pet peeve of mine that most people refer to all primates as monkeys.  In fact the Primate order, which is only one of many orders of the Mammal class (some others include Carnivores, Rodents, and Cetacea (better known as whales)), consists of over 200 species, ranging in size from the mouse lemur (approx. 1 ounce) to the mountain gorilla (400+ pounds), and new species continue to be discovered .

Image copyright John Fleagle (Primate Adaptation and Evolution)

Image copyright John Fleagle (Primate Adaptation and Evolution)

As primates, all of these species share certain characteristics; for example, five digits on each hand and foot, a grasping (prehensile) thumb, good depth perception (stereoscopic vision), and a large brain relative to body size.  But, as the above image illustrates, the Primate order also includes a great deal of diversity.  Not all primates are alike, and not all primates are monkeys!

Other than size, what differentiates all of these species from one another?  Quite a bit, actually.  Different species have different diets (some eat mainly fruit, some eat mainly leaves, and the tarsier is entirely carnivorous), social structures (some primates are mostly solitary, others live in large groups), mating systems (chimpanzees are highly promiscuous, whereas marmosets and tamarins are mostly monogamous), and degrees of intelligence/behavioral complexity.  Some of it is geographic, too.  Lemurs, for example, are found only on the island of Madagascar, off the east coast of Africa, while monkeys are divided into two primary taxonomic groups: the “Old World Monkeys” (Cercopithecoidea), which are found in Africa or Asia, and the “New World Monkeys” (Platyrrhini), which are found in Mexico and Central & South America.

Clockwise from top left: Squirrel monkey (new world), Mandrill (old world), Capuchin (new world), Japanese macaque (old world)

Clockwise from top left: Squirrel monkey (new world), Mandrill (old world), Capuchin (new world), Japanese macaque (old world)

Gladys’s question, though, is about gorillas, and gorillas are apes, a subgroup of the Primate order that also includes gibbons, orangutans, chimpanzees and bonobos, and humans.

So how do you tell an ape from a monkey?  The easiest way is to look for a tail.  While all monkeys (with the exception of Barbary macaques) have a tail — some of them prehensile — none of the apes does.  This is far from the only difference between the two groups (I have listed a few more below in case anyone is curious), but it is the most obvious, and an excellent example of how pop culture so often gets primates wrong.

"Look Ma, no tail!" Curious George isn't a "good little monkey" after all.

“Look Ma, no tail!”
Curious George isn’t a “good little monkey” after all.

Additional differences between monkeys and apes:

1. Apes are adapted for suspensory locomotion — hanging from beneath tree branches rather than walking on top of them — which means they have a broad chest, arms that are longer than their legs (in monkeys arm & leg length is roughly equal), and a highly flexible shoulder joint.

2. Apes tend to rely more on vision than smell.  New World monkeys in particular have a better olfactory sense than apes, but they tend to have dichromatic rather than trichromatic vision.

3. Apes tend to be more intelligent than monkeys.  They also mature more slowly, and tend to live longer.

Q: At what point did tool use first become a primate characteristic?    –Diana, Chicago

A: In the not too distant past, it was thought that only members of our own species had the cognitive sophistication to make and use tools.  We now know, however, that chimpanzees, orangutans, and gorillas all make and use tools in the wild*.  Moreover, in 2007 researchers documented finding stone tools of the type chimpanzees use to crack open nuts that date to 4,300 years ago, suggesting that tool use in apes is not a recent innovation (though whether it was characteristic of the last common ancestor of chimps and humans 6 million years ago, or possibly the last common ancestor of all the living great apes 15-20 million years ago, remains unknown).

Chimpanzee nut-cracking

Chimpanzee nut-cracking

In the hominin lineage — that branch of the primate evolutionary tree on which the genera Australopithecus and Homo lie —  the earliest stone tools date to approximately 2.6 million years ago, and are found in east Africa.  Production of these Oldowan tools, named after Olduvai Gorge in modern-day Tanzania, in which they were found, is most often attributed to Homo habilis, one of the earliest members of the genus Homo.  It’s worth noting, however, that circumstantial evidence suggests that Australopithecus garhi, a contemporaneous species in east Africa, may also have been a tool user.

*Capuchins — New World monkeys notable for having a brain to body size ratio comparable to a chimpanzee — are also capable tool users in the wild.

Q: Please explain this: 

-Noah, Madison

A: That, of course, is one of natural selection’s most fabulously awesome achievements: the star-nosed mole.  It is fabulous.  And awesome.  So awesome, in fact, that the only way I can do it justice is to offer you a very short list of facts about it, and a video so you can see it in action.

Star-nosed mole facts:

1. The mole’s snout consists of 22 tentacles, which are used as a touch organs.  Each tentacle has over 25,000 sensory receptors known as Eimer’s organs, which enable the mole to forage faster and more efficiently than any other mammal on Earth.

2. They smell underwater by blowing bubbles.

Check the little guy out here (the real magic starts at 1:00):

ARKive video - Star-nosed mole - overview

And that’s it!  I hope you’ve all enjoyed this first installment of Ask the Biological Anthropologist!, and please keep the questions coming.

Hamadryas and Pangolins and Mastodons, oh my!

As mentioned in my very first blog post, one of the reasons I started with this endeavor is that I have a lifelong love of learning.  Education is an ongoing process, and knowledge, whether it is put to practical use or sought simply to satisfy personal curiosity, is a fantastic thing.  Imagine my excitement, therefore, when I learned of the 2014 Mammal March Madness competition run by Dr. Katie Hinde, an assistant professor of Human Evolutionary Biology at Harvard University.

Picture adapted from art by Tracy A. Heath, Matt Martyniuk, Sarah Werning, via Phylopic! As seen on the blog Mammals Suck…Milk!

This is my kind of pedagogy!

The competition first caught my attention, I must admit, simply because of its enthusiastic mention of mammals (note the exclamation point in the title of her blog post).  As I’ve said before, I have a rather large soft spot for animals, and am easily persuaded to investigate stories or headlines that promise some kind of faunal component.  The more I thought about it, though, the more I began to admire the ingenuity of this exercise.  Here’s why:

1. It brings science to the masses, and makes it fun.

The NCAA March Madness tournament is ubiquitous at this time of year, and the popularity of bracket competitions is continuing to increase.  ESPN even has its own “resident bracketologist,” which at first glance sounds like a job title as realistic as “space smuggler.”

Although it turns out space smugglers are not only real, but fairly common.

Taking advantage of the pervasiveness of this cultural phenomenon to educate people about science is brilliant.  It not only puts the lesson in a familiar context, but appeals to people’s natural competitive instincts by turning it into a game. And for those like me, who are already interested in science but not well versed in basketball, it provides a different kind of learning opportunity: I now understand the nature of “seeds” and know how to fill out a bracket.

2. It only looks simple.

You may be thinking, “If I want to play I just need to look at the bracket and pick some winners, right?”  Technically you’re correct, but as with the NCAA bracket your chances of winning are much better if you make educated picks.  And in this case, that means you need to know not only what an animal is (Hinde’s bracket, as seen below, has an entire division entitled “The Who in the What Now” that is populated by little-known species), but where it lives and how it lives.  

Hamadryas Baboon. Image from Arkive.org.

Pangolin. Image from Arkive.org.

Mastodon. Image from National Geographic.

A laundry list of questions quickly emerges:  How big is it?  How fast is it?  What kind of “weaponry” or defenses does it have?  Does it live by itself or in a group?  To make things even more exciting, the simulated battles in the early rounds of competition take place in the natural habitat of the higher-ranked seed — Hinde calls it the “home court advantage” — but battle locations are randomized at the level of the Elite Eight and beyond.  Could a pack of hyenas triumph over an orca?  Probably, if the battle were to take place on land!

Even with my more-extensive-than-average background in biology, I found myself diving into Wikipedia articles and professional journals to find the information necessary to assess each species and select winners.  Among other things, I’ve learned in the past week that Hamadryas baboons can amass in troops as large as 800 individuals, that the bowhead whale has a layer of blubber that can be 17-19 inches thick, and that, despite its name, the godzilla platypus (which lived 5-15 million years ago) is only twice as large as a modern-day platypus.

Like this.  But 3 feet long!  Image from National Geographic.

3.  It’s memorable.  

I mean this in a dual sense.  First, because it’s happening outside the classroom, the learning that occurs during the Mammal March Madness tournament is likely to be contextualized differently than information read from an assigned textbook.  And while it’s difficult to say whether any knowledge about animals that’s gained through participation in the MMM bracket game will be more likely to be retained due to its association with a cultural touchstone, it’s a form of education that I wholeheartedly endorse (see: my goal to someday teach a Primates & Pop Culture class).

Second, the game itself is memorable.  This may be an especially important factor for kids, or those who are kids at heart: you play, you learn, and then you will want to play again (and learn about a new set of mammals) next year.  It’s the circle of life learning!

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The Mammal March Madness tournament is now underway, and you can follow #2014MMM on Twitter to keep track of all the live action.  I have the Saber-toothed Cat going all the way, but anything can happen in this mammal-eat-mammal world!

The Saber-toothed Cat. 850 pounds. 12-inch canines. Good luck.

Evolution: this time it’s personal

Usually if I’m thinking about evolution, it’s in a broad academic sense, wondering how the pressures of natural selection have shaped certain characteristics in humans or other species over thousands or millions of years.  Rarely do I stop to think about my own personal relationship with the evolutionary process, or about how my membership in the taxonomic group Homo sapiens predestines me to particular biological challenges.

But now that I am — SPOILER ALERT — in the midst of my first pregnancy, the realities of my evolutionary history are hitting me fast and hard.  I’m suddenly distinctly aware of the physical and physiological difficulties of maximizing my reproductive fitness.  Though it means I am successfully playing the evolutionary game, it turns out that in practice, passing on my genetic material  to future generations is much more of an ordeal than I had anticipated!

Of course, this is not an entirely new revelation for me.  As a student of anthropology I have long been taught that the evolution of bipedalism combined with increases in brain size substantially increased the difficulty of human birth by necessitating a larger head to pass through a narrower pelvic outlet than is seen in chimpanzees and other primates.  In order to enter the world, a human baby must engage in a series of gymnastic maneuvers through the birth canal, and women everywhere — regardless of their knowledge of evolutionary history — know that it’s no exaggeration to say that the process of labor and childbirth is a difficult one.

Image copyright Karen Carr, Smithsonian Institution

Left: chimpanzee infant head size relative to pelvic opening; Right: human infant head size relative to pelvic opening

But the seemingly endless weeks of overwhelming fatigue, constant nausea, headaches, and light-headedness that characterized my first trimester and have now continued into my second have got me thinking about the challenges of reproduction beyond the context of childbirth (still months away), and for the first time I find myself envious of the seemingly easier reproductive pathways that have been favored in other lineages.

For example, my hormone-drenched brain has recently been wondering what’s so great about internal gestation.  After all, it’s not the only viable reproductive strategy.  And with all the discomforts I’m experiencing as a result of the baby growing inside me, I can’t help but fantasize about how much more pleasant things would be if I could emulate a bird and bring forth life simply by sitting on an egg.  Seems ridiculous, right?  Perhaps, but when it comes down to it, some of my favorite activities — reading, crossword puzzles, jigsaw puzzles — are best accomplished while seated, and I’ve gotten  quite good at it over the years.  Moreover, external gestation would go a long way toward evening the sexual division of labor associated with reproduction.  My husband and I, equally capable of egg-sitting, could share incubation duties (as in most species of penguin), and neither of us would need to be subjected to back pain, heartburn, or any of my other aforementioned symptoms.

In a similar vein, I find I have a newly disdainful attitude toward the functionally useless nipples of men.  I am perfectly aware that males have nipples only because of the default human developmental trajectory; selection has had no reason to eliminate them.  Yet I recently found my train of thought wandering along rather critical tracks:
“Come on selection, can’t we step things up a notch?  The equipment is more or less in place — let’s follow the example of the Dayak Fruit Bat* and put it to use!”

And what about sex determination mechanisms?  In humans, sex is dictated by the chromosomal combination that results when sperm randomly meets egg: females have an XX genotype while males are XY.  But in some species, primarily in the reptile class, sex determination is temperature-dependent.  Alligator eggs incubated at or below 86°F produce females, while those incubated at or above 93°F yield males.  In still other cases, both chromosomes and temperature are involved, and “certain incubation temperatures can “reverse” the genotypic sex of an embryo.”

Image copyright Laura Dewan, University of Hawaii

Now, I’m not saying that I want to choose the sex of my baby.  In fact, my husband and I have decided not to find this out pre-delivery.  But I’m sure that some couples, given the choice, would gladly renounce their evolutionary heritage in order to gain this ability.

As for me, I have no doubt that I am oversimplifying.  The seeming perks of other animals’ reproductive pathways must surely be counterbalanced by their own set of drawbacks.  And, as my mother reminded me, I should keep certain broad facts in mind:
1. Memories of my discomfort during pregnancy will fade over time
2. It could always be worse — the road to reproductive success for a female elephant begins with a 22-month gestation!

Kudos to you, Madame Elephant.  Kudos to you.

newborn-elephant

*The Dayak Fruit Bat example may not have been part of the original train of thought recounted here.

The Full Spectrum

Let’s talk for a moment about the special status that humans have in the natural world — or rather, the special status that we often accord ourselves.   Though most people no longer think of the world in terms of Aristotle’s scala naturae, we still have a tendency to consider ourselves unique, separate from and far more advanced than the rest of the animal kingdom.  And it’s true that humans do lots of things that other animals don’t: we  build cities, we fly airplanes, we write books (and blogs) and create works of representational art.  As a species, our accomplishments are substantial.

But if there’s one thing that modern ethological research has demonstrated, it’s that many of our abilities and behaviors can be found at least to some extent in other species, especially in the primate family.  Since Jane Goodall’s early observations of tool use among wild chimpanzees at Gombe Stream National Park led her academic adviser, paleoanthropologist Louis Leakey, to comment that “now we must redefine tool, redefine man, or accept chimpanzees as human,” several once-vaunted hallmarks of humanity have been eliminated from the list of defining characteristics that separate Homo sapiens from other species.  Primatologists have provided evidence that chimps hunt, participate in campaigns of organized aggression toward neighbors, maneuver for status in Machiavellian ways, and reconcile following conflict.  Research on language use by great apes similarly suggests that in many cases the difference between humans and our closest relatives is one of degree rather than kind.

The existence of this behavioral spectrum  has been illustrated yet again in the new issue of the journal Current Biology, in which anthropologists Richard Wrangham and Sonya Kahlenberg report that young chimpanzees in Kibale National Park in Uganda carry and play with sticks in a manner similar to children with dolls.

chimpanzee-stick-doll

A female chimpanzee holds a “stick doll” between her leg and body. (Sonya Kahlenberg)

Examples of play among juvenile animals are not new.  But it is intriguing that these researchers identified a sex-based bias in their data: as in humans, female chimpanzees were more likely to carry stick dolls.  Does this mean that young females are simply practicing for the future by imitating the mothering behaviors they observe within a group?  Or might it suggest innate psychological differences between young male and female chimpanzees?  In either case, how do these data pertain to studies of sex/gender differences in humans? Nature/Nurture debaters, start your engines!!

Whatever the long-term implications of this new study, it serves to illustrate that the more we learn about the behavior of other animals, especially apes, the less reason we have to consider ourselves wholly separate from them.  Of course, this isn’t to say that you should expect to see orangutans in Indonesia building treehouse cities anytime soon.  But try not to be too taken aback the next time you read a story about a wild animal doing something “surprisingly” human-like.
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To read more about play-related sex differences in non-human primates, click here.  And don’t forget to subscribe to The Tinkering Primate if you want to get emails about new posts.  Just click the “Yes Please” button underneath the chimpanzee on the site banner.

The Basics

If I had to choose three adjectives to describe Darwin’s theory of evolution by natural selection, they would be:

1. Elegant
2. Powerful
3. Simple

Yes, simple.  Although evolutionary biologists now know that forces other than natural selection (e.g. genetic drift and gene flow) can cause populations to evolve, Darwin’s theory still provides the basic framework for understanding the history of life, and explains much of the variation we see on our planet.  And it does it with just three premises (hence my simplicity claim).  Any student who passes through my Introductory Biological Anthropology course can recite them for you:

1. Individuals in a population vary
2. Variation is heritable
3. Because of this variation, certain individuals are able to survive and reproduce more successfully in a given environment than others.

That’s it.  Those three premises are all that is required to understand the idea of evolution by natural selection.  So why is there so much confusion and misunderstanding surrounding Darwin’s ideas?  Why, during last year’s celebration of the 200th anniversary of Darwin’s birth and the 150th anniversary of the publication of On the Origin of Species, did a poll reveal that only 4 out of 10 Americans believe in evolution?

The ongoing evolution vs. creationism/intelligent design battle aptly demonstrates that religious background plays a significant role.  But in some cases, people may not believe in evolution by natural selection because they don’t understand the process.  Many misconceptions abound simply because people haven’t been properly introduced to the Darwinian basics.

So in the spirit of defending evolutionary thought, I want to share a few  clarifications that I have found useful when introducing students to Darwin’s theory of evolution by natural selection.

  • The third premise — that because traits vary, certain individuals are able to survive and reproduce more successfully in a given environment than others — is often abbreviated to “Survival of the Fittest.”  But this shorthand only works if people understand that the term “fit” is relative.  Individuals that are highly fit (i.e. able to produce many offspring) under one set of environmental circumstances are not necessarily the “fittest” under different conditions.  In other words, changes in an organism’s habitat can turn underdogs into winners and vice versa.  This is key.  Remember it.
  • Natural selection and Evolution are not terms that can be interchanged at will.  As I tell my students, natural selection is a process that affects individuals, but it is populations that evolve as fitter individuals reproduce more successfully and pass on beneficial trait variations.
    A quick example: Among a population of tree frogs, bright red individuals who are more visible against the green backdrop may be subject to higher predation and die before they reproduce.  As a result, fewer “red color” genes will be passed on to subsequent generations and over time the population will have higher frequencies of the beneficial-for-camouflage “green color” genes.
  • Humans (aka Homo sapiens) are not the pinnacle of evolution.  In fact, there is no pinnacle of evolution, because natural selection is not a unidirectional process in which organisms become “better” in an absolute sense (see the above point about underdogs).  Humans are simply the most recently evolved members of one particular primate lineage.  And, despite what many of my students think, most experts agree that we are still evolving.

That’s the end of today’s lesson.  Now go and explain these premises to everyone you know and maybe, just maybe, we can top 50% in the next Gallup poll.