Science & Nature Archive

Friday, March 17, 2017

Understanding Evolution - How Humans and Apes Fit Into the Tree of Life

I came across a question on Quora the other day that seemed to reflect a common incomplete understanding of evolution, If it took 5 million years for today´s humans to evolve from the apes, how long time did it take for today´s apes to evolve from their origin?. There are a few issues with that question, but rather than enumerate them all here, I'll just jump into the explanation, which will hopefully make it clear as we go. The one thing I'll say up front is that we diverged from chimps & bonobos more like 6 million years ago, not 5 million.

It all depends on what perspective you want to take, and which starting point you want to go with. When people bring up the 6 million years for humans to evolve from apes, what does that really mean? Take a look at this diagram:

Hominid Evolutionary Tree
Click to Embiggen
Image Source: The Open University - Studying mammals: Food for thought


That's one probable evolutionary tree for us over that time (the exact details are subject to debate). Notice how bushy it appears. Populations kept on splitting and splitting and splitting, and most of those species ended up going extinct. We're the only surviving members of that lineage (though Neanderthals nearly made it to the present day). But, if you wanted to ask, how long did it take for humans to evolve, where would you pick as your starting point in that diagram? It just happens to start with Orrorin tugenensis, but that's only because that's where that artist decided to start it. They could just as easily have started with Ardipithecus ramidus, and you could say it took us 4 million years to evolve from that. Or, they could have skipped ahead and started at Homo habilis, and you could say that it took us 2 million years to evolve from that. Or, you can notice that Australopithecus boisei and us are pretty distant cousins on that tree. If A. boisei had managed to not go extinct, or to have left descendants that kept on evolving into some new species, there might be another ape alive right now more closely related to us than chimps and bonobos. So, then we might be saying that it took us 3 million years to evolve from apes. But it wouldn't be anything different about how we evolved - it would just be the fact that we had a still living closer cousin to compare ourselves to. (Note that that terminology is a bit misleading, as you'll hopefully understand after reading this full entry - we are simply apes ourselves.)

Here's another diagram, this time including the still surviving great apes, but not showing all the ancestors or extinct species from side branches that died out:

Ape Evolutionary Tree
Click to Embiggen
Image Source: BOSCOH.com - Milestones of Human Evolution from Paleontology & Bioinformatics


That's where the 6 million year number comes from. It means that 6 million years ago, there was a population of animals whose descendants would eventually become chimps, bonobos, and humans. It was the last common ancestor of us three surviving species. It took each of our species 6 million years to evolve from that population. But recall the branching pattern from the previous diagram. It wasn't a straight line from that population to each of us species that's still around. It split and split and split in a bushy pattern. In the lineage that led to us, only one species survived to the present - us. In the lineage that led to chimps and bonobos, those two species survived to today.

And you don't have to pick just chimps and bonobos. If you look at gorillas, our common ancestor with them was alive roughly 8 million years ago. So, it took 8 million years for gorillas to evolve from that ancestor. It took chimps 8 million years to evolve from that ancestor. It took bonobos 8 million years to evolve from that ancestor. And it took us humans 8 million years to evolve from that ancestor. Chimps, bonobos, and us share a common portion of that 8 million years. Chimps and bonobos alone share an even longer common portion. It would be similar to asking, how many generations did it take to get from your great-grandparents to you, or to your brother, or to your cousin, or to your second-cousin? In all cases, it would be three generations. For you and your brother, you'd share most of that lineage, starting with your great-grandparent, then your grandparents, and then your parents. With your cousin, you would only share your great-grandparents and grandparents. And with your second cousin, it would only be your great-grandparents. There are a lot more greats than that considering our evolutionary history, but it's the same concept. We share more of our lineage with chimps and bonobos than with gorillas. And we share more with gorillas than with orangutans. And we share more with orangutans than with non-apes.

If you want to go further and ask how long it took for apes to evolve, it really depends on how far back you want to go. Here's another diagram:

Primate Evolutionary Tree
Click to Embiggen
Image Source: ResearchGate


Now, we get into a problem of semantics. In language, apes have a name to describe them as distinct from monkeys. But we're not really a completely distinct group. To have a distinct group in classifying these types of things, all members of that group should share a common ancestor that no other group can claim in its ancestry. Apes have such an ancestor around 20 million years ago. The only descendants of that specific animal are apes. But monkeys don't have that type of unique common ancestor. There's no single ancestor of 'monkeys' that isn't also an ancestor of apes. We're not two separate groups. Us apes are really just a specialized subset of monkeys without tails. But, if your question is just when 'apes' first appeared, then like I already said, the last common ancestor of all apes was alive around 20 million years ago.

But why stop there? When biologists say that all life on earth is related, they mean it. All life on earth shares a common ancestor. If you go back far enough, you can find our last common ancestor with chipmunks (~90 million years ago), or with a triceratops (~320 million years ago), or with a goldfish (~432 million years ago), or with an apple tree (~1.6 billion years ago), or even with the streptococcus bacteria that may have given you your last sore throat (~4.3 billion years ago). So, if you want to start at the beginning, you have to figure out when our earliest, earliest single celled ancestors were alive. The problem is that it's hard to find evidence of things that nearly inconceivably ancient, but it was probably more than 4 billion years ago. So, in that sense, it's taken humans over 4 billion years to evolve. It's take starfish over 4 billion years to evolve. It's taken e. coli over 4 billion years to evolve. It's taken oak trees over 4 billion years to evolve. Etc. Etc. Every organism alive is the end result of all that evolution leading up to where it is now.

Complete Evolutionary Tree
Click to Embiggen
Image Source: evogeneao Tree of Life


So to summarize, it's taken chimps, humans, and bonobos roughly 6 million years to evolve from our last common ancestor. It's taken all of us apes as a whole roughly 20 million years to evolve from our last common ancestor. You can keep going back in our ancestry until somewhere more than 4 billion years ago to the first life, that was the ancestor of everything alive today.

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There are some really good trees of life and similar type pages to play around with. Here are a few (I already linked to one above, but it's worth repeating). They mostly include only the tips of the tree for organisms that are still alive. So, you won't necessarily be able to find an Australopithecus or a Tyrannosaurus, but even just sticking to living animals, it's a huge, huge tree.

 

Sunday, February 12, 2017

Understanding Evolution - Balancing Selection Pressures, Or Why All Features Are Tradeoffs

Gazelle & Cheetah DioramaTo celebrate Darwin Day, I'm going to recycle a recent Quora answer about evolution. Somebody had asked, Why would a gene that makes a gazelle slightly faster, but still much slower than a cheetah be favored by evolution?. Here's my answer.

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Because everything in life is a trade-off, and cheetah attacks aren't a gazelle's only concern.

Running faster comes at a cost. In particular, it means bigger or stronger muscles to be able to propel yourself faster. Bigger muscles take more food to grow, and more food to maintain. So, the fastest gazelle is also the most likely to starve in times of scarcity. And it's also putting more of it's food resources into those muscles instead of reproduction and/or nurturing young, and may end up not having as many offspring / surviving offspring as a slightly slower gazelle.

And gazelles have other predators besides cheetahs. One in particular is so efficient that it doesn't really matter how fast a gazelle runs - our bullets are faster. And which animals do trophy hunters target? The biggest and most impressive. It's already been documented that trophy hunting has led to bighorn sheep having horns that aren't so big (Phys.org - Intense trophy hunting leads to artificial evolution in horn size in bighorn sheep), and that size limits in fishing has led to smaller fish (Phys.org - Intensive fishing leads to smaller fish). I don't know if gazelles have been studied in this manner, but I wouldn't be surprised at all if human hunting had strong selection pressures on their sizes.

There's this whole complex network of selection pressures acting on gazelles (and all other organisms). Evolution has to balance (metaphorically since evolution isn't conscious) an organism's strategies to dealing with these pressures, and can't focus on optimizing completely for one selection pressure if it means compromising too much on other ones. So, cheetah attacks are one pressure on gazelles, and this particular pressure pushes gazelles to be faster. So, evolution pushes them to be fast enough to greatly lower their likelihood of being caught by a cheetah. But going even faster would only reduce that risk slightly, and at the cost of hurting the gazelles chances of survival/reproduction in other ways. So, gazelles are fast enough, and there's no reason to waste their limited food resources on even bigger muscles, when they could be using those resources for other activities, or even just being smaller so that they don't need as much food.

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If you want to look at it another way, it's like wondering why everybody doesn't have a Ferrari. Sure, Ferraris are fast, but they're also expensive, use a lot of gas, and have many compromises that make them less than practical everyday drivers. Evolution could make gazelles faster, but only by compromising them in other ways.

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To add one more thing - the reason gazelles just have to be reasonably fast, but not as fast as or faster than a cheetah, has to do with the way attacks actually play out in real life. As Brian Dean pointed out in his answer, it's not like a track race, where the fastest organism is the winner. Cheetah's are only sprinters, with limited stamina. Gazelles are keeping a lookout for cheetahs already, trying to make sure the cheetahs don't get too close. The usual result is that the cheetahs can only get so close before starting their sprint, meaning the gazelles have a head start. The gazelle only needs to be fast enough that it can avoid the cheetah until the cheetah gives up, which is still pretty fast, but a good deal less fast than a cheetah. And an extra few miles an hour on the gazelle's top speed is a sizable percentage difference in how much more time it has to evade the cheetah.

Image Source: Wikimedia Commons

Sunday, February 12, 2017

Happy Darwin Day 2017

I'm cheating. This is mostly copied from last year with just a few updates.

Darwin's BirthdayToday is Darwin Day, the 208th anniversary of Charles Darwin's birth. To quote one of my previous Darwin Day posts, Charles Darwin was "the man who presented evolution in such a way and with sufficient evidence that it became obvious that it was the explanation for how life developed on this planet. Others had ideas of transmutation before Darwin, and Alfred Russel Wallace even came up with a theory of natural selection very similar to Darwin's at around the same time, so it's apparent that humanity would have eventually recognized how evolution works. But Darwin's genius in presenting all the evidence for evolution in the way he did certainly gave the field a huge head start."

If you want to see if there's anything specific going on in your neck of the woods, you can check out the list of events at DarwinDay.org, or my recent post. I couldn't find anything for Wichita Falls again this year. And I never did watch Inherit the Wind last year, so maybe I'll be able to talk my family into it this year.

To celebrate Darwin Day on this site, I'm going to provide links to a few of my previous entries. This first set of links is entirely to entries specifically relevant to Darwin or written just for Darwin Day.

And while I write way too much about evolution to list all of my evolution entries, here are a few highlights since the previous Darwin Day:

Friday, February 10, 2017

Upcoming Darwin Day Events

Darwin's BirthdayFebruary 12th is Darwin Day. Many organizations are planning events for this weekend to celebrate. You can check DarwinDay.org for events close to you. Here are a few from across Texas and Oklahoma (well, at least the ones you can still make it to - there were some events earlier this week).


Austin, TX - Center for Inquiry: Darwin Day 2017
Feb 11, 12pm - 5pm

DarwinDay.org Details
Official Event Page
Facebook Page

"FREE event to celebrate the 208th anniversary of Darwin's birth. There will be something for everyone--fun learning activities for children and teens, fascinating lectures and trivia contest for adults, and professional development credit for teachers."


Tyler, TX - Darwin Day Tyler 2017
Feb 10, 5pm - 7pm
Feb 11, 10am - 4pm

DarwinDay.org Details
Official Event Page
Art & Seek Details

"This year's Darwin Day celebration features four main events: two different public science lectures, evolutionary themed video screenings, and a teacher development workshop. There will also be other events for students of all ages at the Discovery Science Place, the University of Texas at Tyler, and Tyler Junior College.

All events are free of charge!"


Nagadoches, TX - Darwin Day at SFA
Feb 10, 1pm

DarwinDay.org Details
Press Release

"The Stephen F. Austin State University Department of Biology will host its second annual Darwin Day program. The program will include a seminar by Dr. Charles Pence, assistant professor of philosophy and religious studies at Louisiana State University. Pence's seminar is titled 'Evolution and Chance: From Darwin to the Pioneers of Statistics in Biology.' "


San Antonio, TX - Trinity University: Panel Discussion on Darwin, Gender, and Race
Feb 15, 5pm - 6.30pm

DarwinDay.org Details

"In celebration of Charles Darwin's 208th birthday join us for a discussion on Darwin, Gender, and Race. Four Trinity professors will provide a panel discussion of Darwin's views on gender and race, the current scientific views on race, and the contributions of minorities and women to evolutionary theory. Following brief introductory comments, the panel will open up for questions from the audience. Also be sure to check out the exhibit at the Coates Library, Third Floor: Who's studying evolution these days? A look at modern scientists; Feb. 8-15."


Norman, OK - Norman Naturalism Group: Darwin Day! Potluck and Speaker: The EVOLUTION of Religion
Feb 12, 5pm - 9pm

DarwinDay.org Details
Official Event Page

"The Norman naturalism Group celebrates Darwin Day 2017 with a pot-luck dinner and speaker. The topic is "The Evolution of Religion". Pull out your best recipe and get ready for some more good eating and good talking."


Thursday, February 2, 2017

Texas State Board of Education Takes a Small Step Backwards on Science Education

TEA LogoAs described in the Austin American Statesman article, Texas education board approves curriculum that challenges evolution, the Texas State Board of Education has approved some troubling language for the state science standards.

If you want to see for yourself the full standards, you can find them here. Here are the four subject to the current controversy:

(3) Scientific processes. The student uses critical thinking, scientific reasoning, and problem solving to make informed decisions within and outside the classroom. The student is expected to:

(A) in all fields of science, analyze, evaluate, and critique scientific explanations by using empirical evidence, logical reasoning, and experimental and observational testing, including examining all sides of scientific evidence of those scientific explanations, so as to encourage critical thinking by the student;

(7) Science concepts. The student knows evolutionary theory is a scientific explanation for the unity and diversity of life. The student is expected to:

(B) analyze and evaluate scientific explanations concerning any data of sudden appearance, stasis, and sequential nature of groups in the fossil record;

(7) Science concepts. The student knows evolutionary theory is a scientific explanation for the unity and diversity of life. The student is expected to:

(G) analyze and evaluate scientific explanations concerning the complexity of the cell.

(9) Science concepts. The student knows the significance of various molecules involved in metabolic processes and energy conversions that occur in living organisms. The student is expected to:

(D) analyze and evaluate the evidence regarding formation of simple organic molecules and their organization into long complex molecules having information such as the DNA molecule for self-replicating life.

You can read a detailed discussion in a report put out by the Texas Freedom Network, Texas Science Curriculum Standards: Recommendations for Dealing with Pedagogical and Scientific Problems (pdf).


The Bad

Yes, the motivation behind these standards really is to promote creationism / cast doubt on evolution. Here are a few excerpts from that TFN report regarding the motivation behind three of these:

[Regarding 7 B] In a final appeal to preserve his proposal, McLeroy stated that the purpose of his standard was to argue against: "...the idea that all life is descended from a common ancestor by the unguided natural processes."
[Regarding 7 G] During the board debate, McLeroy explained that this standard: "...questions the two key parts of the great claim of evolution, which is [sic] common ancestry by unguided natural processes."
[Regarding 9 D] During board debate, Don McLeroy, R-Bryan, explained that the new standard was "basically an origin of life amendment," referencing public testimony provided previously by Ide Trotter, a well-known promoter of intelligent design."

And the history of the first one, 3A, makes it clear that this was compromise language regarding the strengths and weaknesses gambit so popular among creationists.

Moreover, the Board had actually formed an expert committee to review the standards and make recommendations on improving them, and the committee recommended removing these four particular items because "they were vague, redundant or would require too much time to teach" (quoting the Stateseman article). So, the Board is going against the advice of experts to push standards that were originally motivated by anti-science positions.


The Good

The standards aren't actually that bad. All of them could be handled by textbook publishers and teachers strictly keeping to real science, and not injecting any creationism or other pseudoscience. Let's look at them again on a case by case basis.

(3) Scientific processes. The student uses critical thinking, scientific reasoning, and problem solving to make informed decisions within and outside the classroom. The student is expected to:

(A) in all fields of science, analyze, evaluate, and critique scientific explanations by using empirical evidence, logical reasoning, and experimental and observational testing, including examining all sides of scientific evidence of those scientific explanations, so as to encourage critical thinking by the student;

Well, it says specifically 'all sides of scientific evidence'. Creationism is manifestly not science, so this shouldn't be a backdoor for creationism. As far as real science, this is a little overwhelming for a high school biology class. I mean, all sides of the scientific evidence supporting evolution in general could be an entire class unto itself. Even 150 years ago, in The Origin of Species, Darwin had an entire tome full of evidence for evolution, and the evidence has only grown stronger and more abundant since.

Granted, there are different 'sides' within current evolutionary biology - the relative influence of genetic drift vs. natural selection, how much of the genome is truly junk DNA vs. possible other functions, etc. So, teachers could delve into these current topics, but it seems a bit of a deep dive for high school biology.

(7) Science concepts. The student knows evolutionary theory is a scientific explanation for the unity and diversity of life. The student is expected to:

(B) analyze and evaluate scientific explanations concerning any data of sudden appearance, stasis, and sequential nature of groups in the fossil record;

Well, if you're sticking to real science, this is simply a discussion of gradualism vs. punctuated equilibrium, and perhaps some background on taphonomy and taphonomic biases in the fossil record. And that's all a decent discussion to have, showing students the evidence in support of both gradualism and punctuated equilibrium. In fact, there's evidence for both, so it's probably not an either/or discussion, but rather how they represent opposite extremes regarding the rates of speciation, and what might drive the different rates of change. Although like I said above, this is getting pretty in depth for a high school biology class.

(7) Science concepts. The student knows evolutionary theory is a scientific explanation for the unity and diversity of life. The student is expected to:

(G) analyze and evaluate scientific explanations concerning the complexity of the cell.

Well, yes, cells are complex. I remember learning about that back when I was in high school, and making a model stuffed full of organelles. And if you really want to get into the origins of the complexity, symbiogenesis is one of the topics to discuss in the origin of eukaryotes. And there's an entire field of study for abiogenesis, concerning how life first arose. But again, this might be more detailed than most people expect from high school biology.

(9) Science concepts. The student knows the significance of various molecules involved in metabolic processes and energy conversions that occur in living organisms. The student is expected to:

(D) analyze and evaluate the evidence regarding formation of simple organic molecules and their organization into long complex molecules having information such as the DNA molecule for self-replicating life.

Lots of good stuff to discuss here, as well. I'm sure that teachers would at a minimum bring up the Miller-Urey experiment, as well as other more recent experiments that used different conditions thought to be more representative of the early earth. Teachers could start discussions on the RNA World. And of course, there's that whole field of abiogenesis that I already linked to. But like I said for each of the other questionable standards, and like the expert committee said, this is getting awfully detailed for a high school biology class that has to cover all the other standards and curriculum.

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So, it's troubling that these standards were motivated by creationist misunderstandings of science, and that the Board members went against the recommendations of experts regarding the standards. But at least the letter of the standards isn't horrible, and textbook publishers and teachers can stick to real science. I just hope teachers with creationist sympathies don't use these standards as an excuse to teach junk science.


Updated 2017-02-03: Made numerous small changes

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