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Cave Bear

Cave Bear

In the "Jurassic Park" movies, scientists re-created living dinosaurs from fossilized DNA. That's science fiction, but recently, real-life scientists reconstructed the entire genetic code of an extinct cave bear.


Transcript

Decoding an ancient cave bear. I’m Bob Hirshon and this is Science Update.

A two-ton, thirteen-foot cave bear, extinct for ten thousand years, has just experienced a rebirth of sorts. From a tooth and a bone, scientists have recovered its entire genetic code.

Eddy Rubin, director of the Department of Energy’s Joint Genome Institute, says finding genuine cave bear DNA was like looking for a needle in a haystack.

Rubin:

And the haystack were all the other organisms that were living in the bones and in the tooth of this ancient creature. And the needle was the little bit of the ancient creature’s genome DNA, or genes.

They used state-of-the-art computer technology to separate the bear genes from the clutter. Jurassic Park fans should note that they can’t clone a new cave bear, nor can they recover DNA from creatures as old as the dinosaurs. But they do hope to reconstruct the genetic code of Neanderthals, our closest non-human relatives, to better understand how our own species evolved. I’m Bob Hirshon for AAAS, the Science Society.


Making Sense of the Research

Since the structure of DNA was first described over fifty years ago, scientists have scrambled to put together the complete genetic sequence of everything from viruses to humans. It's a huge undertaking for even a living creature – the Human Genome Project, for example, took 13 years to sequence the complete human genetic code, and that was ahead of schedule. Understanding an organism's complete genetic blueprint is as useful to a biologist as the architectural plans for a building are to an engineer, or the programming code for a piece of software is to a computer technician.

But until now, scientists had been unable to sequence the DNA of an extinct organism. The challenge has been collecting it. Like the body itself, DNA begins to decompose at death, so finding intact DNA from a creature that's been extinct for thousands of years is hard enough. But even when small fragments of DNA can be recovered from a fossil, it's mixed up with DNA from all the microorganisms that contaminated the dead body from ancient times to the present. Even the fingerprints of the researchers themselves can leave an overwhelming amount of DNA compared to the tiny traces in a fossil.

The researchers got around this problem through brute force. They decided not to bother trying to purify the cave bear DNA in the sample itself. Instead, they ran the cave bear bone samples, junk and all, through a powerful computer program. The computer scanned all of the genetic sequences for fragments of potential cave bear DNA, and separated the data from all the other genetic codes in the sample.

How did the computer know what to look for? Well, it turns out that bears, including ancient cave bears, are closely related to dogs – they're about 92 percent similar on the genetic level. And it just so happened that scientists have sequenced the entire dog genome. So the researchers programmed the computer to pull out anything that looked like dog DNA. Since dog DNA looks very different from microbial DNA, the computer could spot the dog-like sequences in the cave bear sample.

This technique has its limits. Creatures that are millions, rather than thousands, of years old (like the dinosaurs) are too old and decomposed for this kind of reconstruction. However, there are plenty of more recent specimens to study, including extinct relatives of humans. Candidates include Neanderthals and the Flores Man, an 18,000-year-old offshoot of the human ancestor Homo erectus. Although very human-like, these hominid species aren't direct ancestors of modern humans, but rather are branches of our family tree that hit dead ends and went extinct. By comparing these species to modern humans, scientists may figure out what made our genetic line succeed while others failed.

Now try and answer these questions:

  1. Why is it hard to recover ancient DNA?
  2. How did the researchers solve this problem?
  3. What are the potential uses for understanding complete genetic sequences of ancient organisms – or any organism, for that matter?
  4. Why is it useful to compare modern humans to ancient non-human relatives, like Neanderthals and Flores man?

For Educators

Find out about the sequencing of the complete human genetic code on the U.S. Department of Energy's official Human Genome site.

In the National Geographic News article Prehistoric DNA to Solve Human-Evolution Mysteries? read about how tiny flecks of DNA from the fossils of human ancestors and other primates may shed light on the evolution of modern humans.

The Access Excellence resource Evolution is a comprehensive lesson designed to bring evolution out of the last chapter of textbooks and into the entire biology curriculum.


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