Ancient DNA Gets Older and Multiple Births Get Bigger


Ancient human DNA gets older and older

Big stories involving ancient DNA are popping up nearly every week.  This week’s ancient DNA news is recovery of the oldest human DNA yet, from a Spanish cave chamber called Sima de los Huesos, the Pit of Bones. The aptly named pit provides excellent storage conditions and contains bones from many animals and about 30 humans.

A fossil human skeleton from Spain's Pit of Bones. Credit: Javier Trueba, Madrid Scientific Films

A fossil human skeleton from Spain’s Pit of Bones. Credit: Javier Trueba, Madrid Scientific Films

A thigh bone from the Sima has yielded–not without a struggle–human DNA estimated to be about 400,000 years old. Or call it hominin DNA if you insist on applying the term “human” only to the most recent incarnation and sole surviving member of the genus Homo. Which is to say, us. Although so narrow (and narrow-minded) a definition strikes me and others as a phylogenetic classification error as well as a philsophical and psychological one.

Be that as it may, DNA from 400,000 years ago is undeniably a big deal. But we might get back as far as a million years before too long, as I note in a post I did Tuesday for the Genetic Literacy Project. That’s because technological developments are making it possible to recover and analyze tiny bits of DNA.

DNA is much like the living beings that house it: The older it gets, the more likely it is to fall apart. Fragments are all that remain. Which means that getting complete nuclear genomes from very old material, plant or animal, is a long shot.

The Pit of Bones human DNA is mitochondrial DNA, the small genome–containing only 37 genes–powering the structures that supply a cell’s energy. There’s only one nuclear genome per cell, but there can be thousands of mitochondria. So the chance of finding mtDNA in a fossil is much greater than the chance of finding DNA from a cell nucleus–although it’s nuclear DNA, which contains about 22,300 genes, that we generally mean when we’re talking about “the genome.”

You can find details and speculations about the Spanish discoveries at Carl Zimmer’s always-informative blog The Loom. He answers reader questions too.  The equally informative Ed Yong, writing at The Scientist, is very good here on the technical wizardry that achieved this–I shudder to use such a shoddy and verboten term–“breakthrough.” In Ed’s defense, he didn’t say breakthrough, the top-level ancient DNA expert David Reich did.

But still.


Surprising and mysterious? Not if you’ve been paying attention

There’s a tendency for the media to treat scientific discoveries as surprising and/or mysterious. That’s partly due to an unfortunate truth about journalism, which is that your editor may not be interested unless you can make the case that your piece will startle readers.

Ancient DNA work is often a fine example. From the Pit of Bones we get heds like these: ‘Oldest’ Human DNA Reveals Mystery (Voice of America), Baffling 400000-Year-Old Clue to Human Origins (New York Times), Oldest Human DNA Discovered, Muddles Picture of Our Origin (Time).

This week’s ancient DNA strikes some people as mysterious for the same reason that other recent finds do, for example the 24,000 year-old Siberian boy with what appears to be European ancestry that I wrote about at GLP last week. These discoveries seem surprising in part because many of us are still in the grip of a now-untenable idea: That human evolution (or, actually, evolution of many organisms) is lineal, a straight line of descent.

It is now clear, however, that our ancestors were enthusiastic networkers, driven by wanderlust and the other kind of lust. Just, in fact, like us–although that seems to have come as a shock in some quarters.

John Hawks, everybody’s favorite paleoanthropologist-blogger, uses the Pit of Bones paper to explain why straight-line descent is utterly inadequate to explain what ancient human DNA is telling us. Although he also discusses nuclear DNA, in the quote below Hawks invites you to look at the evidence just on mtDNA. And note that because it’s mtDNA, which is inherited only from mothers, the subtext is a commentary on how women, not men, moved around and mated:

“Humans today descend in part from Neandertals, even though Neandertal mtDNA is gone. Europeans today are largely different from the Europeans of 10,000 years ago, with a massive mtDNA replacement along with the introduction of Neolithic culture, and at least a second later large-scale replacement of genetic diversity. Earlier Neandertals in Europe have different mtDNA diversity than later Neandertals in Europe. Denisova cave [in Russia] was home to an earlier population of hominins with different mtDNA than the later Neandertals who lived there. Mitochondrial DNA has never been a straight line linking earlier and later populations within a single location. Whenever we look at ancient DNA in hominins, the earlier populations have different mtDNA diversity than the later ones.”

I could go on and on, but Hawks is the oracle on this topic.  A must-read if you care about human evolution, and who among us does not?


How to control multiple births

Speaking of human reproduction, a study from the Centers for Disease Control and Prevention, just published in the New England Journal of Medicine, is reporting that fertility drugs are now the most important cause of multiple births.

In the US the proportion of multiple births doubled, from 1.8 percent to 3.5 percent, between 1971 and 2011. Nearly all of the increase was due to fertility treatments, Nancy Shute reports at Shots.

Credit: Dustin M. Ramsey via Wikimedia Commons

Credit: Dustin M. Ramsey via Wikimedia Commons

Used to be that in vitro fertilization was the culprit. (I say culprit because, despite the public oohs and aahs and media gushes over twins, triplets, and especially beyond, the human uterus is designed to hold a single fetus. Non-singleton pregnancies are riskier–sometimes much riskier–for both mother and babes.) Back in the last century, docs often transferred three or more lab-created embryos into a waiting uterus in order to increase the chances that at least one would survive.

The practice of transferring multiple embryos was discouraged in 1998 as a result of new guidelines for ART–one of my favorite acronyms, standing for “assisted reproductive technology.”  Now increases in multiples are being driven by fertility drugs, which stimulate ovulation and often release more than one egg at a time.  But super-ovulation can be better controlled, according to Rachael Rettner at LiveScience, for example by lowering fertility drug doses.

Now that semi-alarming data on lab-created multiple births are backed by the authority of the CDC, the health-watchdog government agency, plus NEJM, the top medical journal, it is pretty safe to predict that ARTists will shortly feel pressure to practice safety measures like these.



One Response to Ancient DNA Gets Older and Multiple Births Get Bigger

  1. Ricki Lewis says:

    Great post! It’s exciting to blog about these discoveries, but they are the bane of textbook writing. I’m working on the 11th edition of my human genetics textbook, and spent an entire month on the chapter about human ancestry, compared to days or at most close to a week for every other chapter. As soon as I think I’m done, another discovery comes along. But I do tend to wait until the analysis reaches nuclear DNA, which usually follows 2 years or so after mitochondrial. Thanks for the terrific summary.