The Mystery of the Missing Chromosome
By CARL ZIMMER - DISCOVER MAGAZINE BLOGS
Added: Sat, 21 Jul 2012 18:16:09 UTC
There’s something fascinating about our chromosomes. We have 23 pairs. Chimpanzees and gorillas, our closest living relatives, have 24. If you come to these facts cold, you might think this represented an existential crisis for evolutionary biologists. If we do indeed descend from a common ancestor with great apes, then our ancestors must have lost a pair after our lineage branched off, some six million years ago. How on Earth could we just give up an entire chromosome.
A close look at our genome and the genome of our close relatives reveals that we didn’t. We just combined a couple of them. Every now and then, chromosomes fuse. This fusion occurs as sperm and eggs develop, as pairs of chromosomes fold over each other and swap chunks of DNA. Sometimes two different chromosomes grab onto each other and then fail to separate. Scientists have observed both humans and mammals with fused chromosomes. Chromosomes typically have distinctive stretches of DNA in their center and at their ends. From time to time, scientists will find an individual that’s short a chromosome, but one of the chromosomes it retains now has an odd structure, with chromosome endings near the middle and other peculiar features.
This might seem like a fantastic mutation–something like a human and a horse being joined into a centaur. Remarkably, however, fused chromosomes are real, and there are surprising number of normal, healthy people carrying them.
If humans and apes did indeed share a common ancestor, then it would make sense that two chromosomes fused in our ancestors. The rise of genome sequencing allowed them to test that hypothesis. They found that human chromosome two bears the hallmarks of an ancient chromosome fusion, with remnants of chromosome ends nestled at its core. In 2005, it became possible to test the hypothesis again, when a team of scientists sequenced the chimpanzee genome and could compare it to the human genome. The chimp genome team were able to match human chromosome two to two unfused chromosomes in the chimpanzee genome.
Stephen Cave - Financial Times Comments
What we really know about our evolutionary past – and what we don’t
- - Ancestors Trail Walk Comments
WALK DARWIN’S TREE OF LIFE ~ 26 AUGUST 2012 - event begins on Saturday 25 August
Liat Clark - Wired.co.uk Comments
Astrophysicists simulate 14 billion years of cosmic evolution in high resolution
Alok Jha - The Guardian Comments
Cambridge scientists claim DNA overlap between Neanderthals and modern humans is a remnant of a common ancestor
- - Science Blog Comments
Why, after millions of years of evolution, do organisms build structures that seemingly serve no purpose?
Charles Choi - CBS News Comments
Four decades ago, in 1972, the Koobi Fora Research Project discovered the enigmatic fossilized skull known as KNM-ER 1470 which ignited a now long-standing debate about how many different species of early Homos existed.
MORE BY CARL ZIMMER
Carl Zimmer - iTunes Comments
Science writer Carl Zimmer and evolutionary biologist Douglas Emlen have teamed up to write a textbook intended for biology majors - free app available for the iPad
Carl Zimmer - Discover Magazine Blogs 13 Comments
The human genome contains about 100,000 fragments of endogenous retroviruses, making up about eight percent of all our DNA
Carl Zimmer - The New York Times 19 Comments
The first goal of the project, known as the Open Tree of Life, is to publish a draft by August 2013. For their raw material, the scientists will grab tens of thousands of evolutionary trees that are archived online. They will then graft the smaller trees into a single big one.
Carl Zimmer - The Loom 6 Comments
A Hot Young Earth: My Answer to the
Annual Edge Question
Carl Zimmer - Discover Magazine 8 Comments
A British family with a bizarre speech deficit has led linguists to FOXP2: a gene that begins to explain how our ancestors acquired language.