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DISPLAYING POSTS TAGGED: evolution (8)

Chimp and human DNA

Author
by Kate C
Publish date
11 October 2011
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Exhibitions about science and technology are notoriously difficult to keep up-to date because those scientists just won't stop discovering and inventing things! Curator Kate Phillips encountered an example of this last week, after someone spotted a discrepancy between two Melbourne Museum exhibitions, Darwin to DNA (2000) and 600 Million Years: Victoria Evolves (2010).

Both exhibitions compare the similarity of DNA between chimpanzees and humans. The earlier exhibition states that there is less than two per cent difference while the more recent exhibition declares a 96 per cent similarity. While the numbers don't seem to agree, they're not necessarily incorrect because they compare different aspects of the genomes.

Face of young adult male chimpanzee. Young adult male chimpanzee.
Image: Frans de Waal, Emory University
Source: Used under Creative Commons CC BY-NC-SA 2.0 from Wikimedia Commons.
 

Kate explains:

"The discrepancy comes about because these two exhibitions were developed ten years apart and the understanding of DNA has changed over that time. In 2001 the draft human genome was published and a final version in 2004. In 2005 the draft chimp genome was published and could be accurately compared to the human one. The percentage similarity that came out of this comparison was 96 per cent. Before this time the similarity was probably based on comparing known genes, and therefore was working with less information."

"However the percentage you come up with also depends on how you make the comparison – on which bits of the genome you compare and that could also account for the discrepancy. If you compare genes, we are more similar, if you include the non-coding sequences, we are slightly less similar. Really 98 per cent and 96 per cent are both indicate great genetic similarity."

Set of chromosomes of a human male. Chromosomes of a human male. Humans have 23 pairs of chromosomes and chimpanzees have 24 pairs.
Source: National Human Genome Research Institute
 

We love that someone noticed this because it means that people are reading exhibition text closely, and keeps us on our toes. It's also, as Kate concludes, a pointed demonstration of "the scale of scientific discovery in the area of genome research over the last ten to twenty years."

Links:

The Chimpanzee Sequencing and Analysis Consortium (2005) 'Initial sequence of the chimpanzee genome and comparison with the human genome' Nature, Vol 437 pp 69-87.  (PDF, 4.3 MB)

Media release from NIH News, 'New Genome Comparison Finds Chimps, Humans Very Similar at the DNA Level' (2005)

Evolving the biggest mouth in history

Author
by Kate C
Publish date
17 August 2011
Comments
Comments (1)

Imagine that your face was articulated so that your jaw could split down the middle and expand sideways until the tips were out as wide as your ears. Imagine that you could move all the bones of your face... not just the soft tissue, but the bones themselves.

Sound bizarre? Alien, even? Yet this is exactly what happens every time a Blue Whale takes a gulp of water. The filter-feeding whales, otherwise known as baleen whales or mysticetes, have feeding adaptations that are unique among mammals. Their intriguing evolutionary history is the subject of Dr Erich Fitzgerald's research, and today he's published a paper that overturns a long-held belief about how the baleen whales evolved.

Blue Whale Illustration of the biggest mouth in history at work. The Blue Whale can expand its mouth to gulp huge volumes of krill-filled water.
Image: Carl Buell
Source: Museum Victoria
 

For several years, he has worked on an extraordinary 25 million-year-old species known from fossils that were found in the 1990s near Jan Juc on Victoria's west coast. Called Janjucetus, this early baleen whale predated the evolution of baleen – the hairy structure used by modern baleen whales to filter tiny crustaceans from the sea. Instead, Janjucetus had the large eyes and ferocious teeth of a hunter.

Erich Fitzgerald with Janjucetus Dr Erich Fitzgerald holding the jaws of Janjucetus with Melbourne Museum's massive Blue Whale skeleton in the background.
Image: Jon Augier
Source: Museum Victoria
 

There are two key changes in the skull that permit the filter feeding of modern whales. The first is a lower jaw that can split down the middle. In humans, the seam (or symphysis) where the two halves of the jawbone meet at our chin is fused, thus our jaws are rigid. In contrast, baleen whales have greatly elongated jawbones that do not meet in the middle. The second change is in the width of the upper jaw; baleen whales have evolved a wide mouth, allowing them to engulf massive volumes of water.

"Previously it was thought that the origins of both features were intimately linked to filter-feeding and that's what differentiated baleen whales from toothed whales and dolphins," explains Erich. His research has just overturned this theory since Janjucetus had a wide upper jaw yet its lower jaw had a tightly connected, immobile symphysis. "So, the loose symphysis is not typical of all baleen whales, it's a later innovation. The earliest baleen whales could not expand and contract their lower jaws so were anatomically incapable of filter-feeding, yet they had these wide upper jaws."

Jaws of Janjucetus The fossilised jaws of Janjucetus, clearly showing the immobile symphysis at the tip.
Image: Jon Augier
Source: Museum Victoria
 

What Erich describes is an elegant example of an exaptation, where a feature evolved to serve a particular function but was later co-opted into a new role. Erich believes that its wide jaw helped Janjucetus to suck in large singe prey items, such as squid or fish, and didn't evolve for filter-feeding at all.

Says Erich, "Charles Darwin reflected upon this in The Origin of Species. He wondered how you could go from a whale that has big teeth like Janjucetus does and catching fish and squid one at a time, to something like a modern Blue Whale that feeds en masse. This is the kind of fossil palaeontologists dream of finding because it shows a transitional form."

"It's an exciting discovery, but actually not as surprising as you might think," concludes Erich. "Evolution by natural selection implies that we should expect to find these kinds of fossils in the rocks." The next question he looks forward to answering is how whales shifted from suction feeding to filter-feeding. "I think we're really close to finding a transitional series of fossils that illuminate this."

Erich's paper about this discovery, 'Archaeocete-like jaws in a baleen whale', is published today in Biology Letters.

Links:

Video: Erich discusses whale evolution

MV News: Ferocious fossil

Dr Erich Fitzgerald

Baleen and toothed whales

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Updates on what's happening at Melbourne Museum, the Immigration Museum, Scienceworks, the Royal Exhibition Building, and beyond.

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