Why did humans lose our tails? Blame a "jumping gene"

 

Most animals have tails, including almost all mammals. For some reason, we humans don’t. This difference has been the source of much speculation among scientists over the years, and many arguments have been made about why we don’t tails.

One line of reasoning goes like this: tails are very useful for animals that live in trees, but once our ancestors came down from the trees and started living on the open plain, they didn’t need those tails any more. But why lose them? Lots of animals don’t live in trees, and they still have tails.

Even among the primates, most species have tails, but chimpanzees, gorillas, orangutans, and bonobos–the great apes–don’t. In fact, one way to tell great apes apart from other apes is by the presence of a tail. We humans are simply great apes without so much hair. Or, as the English scientist Desmond Morris called us in his famous book, humans are “The Naked Ape.”

So why am I writing about this now? Well, in a newly published article in Nature, a group of scientists from NYU, led by Itai Yanai and Jef Boeke, seem to have figured out what made us lose our tails. It’s all due to a piece of DNA that copies itself and jumps around our genome.

It’s a bit geeky, but stay with me and I’ll try to explain.

It seems that sometime around the divergence of the great apes from other primates, about 15-20 million years ago, a “jumping gene” popped into a gene called TBXT in our ancestor. (The B in TBXT stands for brachyury, which means “short tail.”)

The jumping gene here is just a piece of DNA a few hundred letters long*, not really a gene all by itself. But once that jumping gene got into TBXT, it was in just the right position to make the cells in our ancestor produce a shorter version of TBXT. The shortened gene was missing one of its pieces, but it still worked – well, sort of. Our ancestors managed just fine, but they lost their tails.

(Aside: the piece that’s chopped out is called exon 6, for those who really want to know.)

Given that this happened over 15 million years ago, how did the scientists prove their hypothesis? Well, other mammals have the same gene, but they make a longer version. So the authors of the new paper created a version of the TBXT gene in mice that included the jumping gene–and, as predicted, some of the mice lost their tails entirely.

Admittedly, this doesn’t exactly prove that one jumping gene caused us to lose our tails. Without a time machine to take us back 15 million years (with a DNA sequencing machine in tow), we can’t truly prove what happened eons ago. But it’s a compelling story, because we know that our genomes, and those of other great apes, have this unique jumping gene that other mammals lack.

So now we know how we lost our tails. We still don’t know exactly why, though. Some scientists speculate that being tail-less might have helped us to walk upright, or that it might have been better to lose the tails once our ancestors stopped living in trees.

On the other hand, guinea pigs don’t have tails either, and they don’t walk on two legs. And koalas don’t have tails, even though they live in trees. Some of these questions may just have to remain a mystery.

*Technically, the jumping genes in this story are called Alu elements, and they occur all over our genome. Famed geneticist Haig Kazazian, a former Hopkins colleague who passed away just two years ago, explained in a 2004 paper that Alus are a form of “nonautonomous retrotransposon.”

South Carolina lawmaker wants to force Creationism down students' throats

Well, it’s happened again. The great state of South Carolina has demonstrated that when it comes to ignorance of science, its legislators take a back seat to no one. They must have been jealous of Kansas, Louisiana, and Texas.

Last week, SC legislator Mike Fair, a Republican, proposed a new standard for teaching high school biology that encourages teachers to teach alternatives to evolution, by which he means creationism. He's been working on this for months; last spring he tried to pass a law that would have required students and teachers to construct arguments against evolution. After failing to get that through his committee, he has proposed a new law that says: 

“evolution is continually open to and subject to experimental and observational testing.”

Except of course that's not what he really means.

Let’s be clear: Mike Fair doesn’t want evolution to be taught in public schools. Instead, he wants to force students, using the power of government, to adopt his conservative Christian views, which teaches that God created all living things just as they are today, about 6000 years ago (or 4000 years, depending on who you ask). 

Fair has a history of trying to dumb down the teaching of science.  Back in February, he blocked the state education oversight committee from using the phrase “natural selection” in the state science standards. Speaking to the (SC) Post and Courier, Fair said 

“To teach that natural selection is the answer to origins is wrong. I don't think it should be taught as fact.” [Mike Fair, S.C. legislator]

Ignorant barely begins to describe this statement. Mike Fair clearly doesn’t have the faintest grasp of biology or genetics. He’s the last person that anyone should want to weigh in on science standards. His behavior goes far beyond mere ignorance, though: not only is he wrong, but he wants to use the power of the state to impose his religious views, under the guise of science, on every student in South Carolina’s schools. No wonder South Carolina is perennially ranked near the bottom of the country in public education. 

I have a confession to make. I grew up in South Carolina and went through the public schools there, from kindergarten right through high school. I met lots of guys like Mike Fair: popular, plays on the football team, student body president. These guys are usually bullies (we've all seen the movie), and that’s just what Fair is demonstrating now: he wants to bully every teacher, and every child, into listening to his ignorant views of science. I’ve no doubt that if Fair could require prayer in every school — Christian prayer, that is — he’d do that too. I grew up surrounded by this kind of nonsense, but I didn't speak up then because I would have been ostracized. Well, I'm speaking up now. 

Fair and his colleagues in the Republican-dominated S.C. House of Representatives argue that no, they aren’t forcing teachers to teach creationism — they just want to teach the controversy. Equally appalling is the position of the S.C. Superintendent of Education, Mick Zais, who agreed with this sentiment, saying: 

"We ought to teach both sides and let students draw their own conclusions."

No, you shouldn't. There is no scientific controversy about evolution. Evolutionary theory is based on an enormous edifice of facts, with literally tens of thousands of scientific papers providing evidence to support it. There is no competing theory out there.

Ironically, three years ago Fair introduced a bill to prevent the imposition of Islamic-based Sharia law in South Carolina. He justified this by saying 

“A growing concern is the immigration of people who are accustomed to their religion and their civil laws being inextricably connected. For those newcomers to our state, this bill will be helpful to them as they are assimilated into our culture maintaining complete freedom to worship as they please."

Reading this sent my irony meter way into the red zone. Let me see if I understand: Mike Fair doesn’t want religion and civil laws to be “inextricably connected” — but he does want to require that public, state-funded schools teach his religious view of the creation myth. I guess what he meant to say is that it’s okay to mix religious fundamentalism and civil law, as long as it’s Mike Fair's brand of Christian fundamentalism.

South Carolina doesn't need its own set of science standards, nor does Texas, Louisiana, or Kansas. The laws of science don't change when you cross state lines or national borders. Allowing politicians to set science standards is a recipe for disaster, and is one reason why the U.S. continues to lag the rest of the world in science education—as South Carolina has once again demonstrated.

Evolution bugs me

Cover illustration for the forthcoming textbook
by Carl Zimmer and Douglas Emlen

This is a real photo.  It's the cover for a new biology textbook by Carl Zimmer and Douglas Emlen.  The photo shows three "walking leaves," or leaf insects, which have evolved to look just like the leaves around them.

These little bugs must be one of the best examples of natural selection ever.  The more closely they resemble a leaf, the more difficult it is for predators to find them.  Over the millenia, the bugs who evaded predators survived better, and eventually walking leaves looked like they do today.

In fact, these guys have been around for a long time, at least 47 million years, according to a fossil discovery published in 2006.  That paper described a fossilized leaf insect found in an ancient lake bed in Germany, which looks remarkably similar to its modern descendants.

Yes despite this amazing evidence, only 40% of Americans believe in evolution. State legislatures continue to pass bills attempting to teach children the creation myth that the world was created less than 10,000 years ago.  Just last week, a committee in Alabama's legislature proposed a new law that would give students credit for courses on creationism.  The bill's sponsor, local legislator Blaine Galliher, was very clear about his motivation:

"They teach evolution in the textbooks, but they don't teach a creation theory…. Creation has just as much right to be taught in the school system as evolution does." 

I feel sorry for the students in Alabama whose education might suffer thanks to the ignorance of their lawmakers.  But there is some hope: in the national poll showing that 40% of Americans overall accept evolution, among young adults that percentage was 49%, while among over-55 set it was just 31%, as shown in this graph:

So perhaps in another hundred years or so, Americans will start to "get" evolution.  But it might take longer: apparently 18% of Americans still believe that the sun revolves around the earth.  So it's taken us 500 years to get to 80% for that one.