Tausch: Muskeln gegen Gehirn
Endlich ist ein molekularer Unterschied zwischen Mensch und Schimpanse gefunden, in der Verteilung der Energie an Gewebe.
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Bei aller Hochachtung für unsere Cousins, die Schimpansen, die vieles können, was wir können, Werkzeuge gebrauchen etwa und sogar herstellen – kognitiv können sie uns doch nicht das Wasser reichen, Menschen waren auf dem Mond, Schimpansen sind in ihren Wäldern geblieben!
Aber warum, wo liegt die Differenz und wie kam sie zustande? An der Antwort ist selbst unsere Kognition bisher gescheitert: Über das ganze Genom hinweg sind die Differenzen marginal, auch bei einzelnen Genen hat sich kaum etwas gefunden: Wir haben eine besondere Variante von Foxp2, sie wird mit der Sprache und dem Sprechen in Verbindung gebracht, und wir haben eine Variante von Myh16, das ist ein Gen der Kaumuskulatur, das bei uns schwächer ausgebildet ist, das gibt die Freiheit zum Reden.
Großer Unterschied im Metabolom
Zudem gibt es noch Kandidaten in jenem überwiegenden Teil des Genoms, das nicht für die Produktion von Proteinen zuständig ist, aber wirklich etwas gefunden hat man nicht, vor allem dort nicht, wo der Unterschied so eklatant ist, in den Gehirnen: Die der Schimpansen haben 450 Kubikzentimeter Volumen, unsere haben dreimal so viel (mit hohen Schwankungsbreiten zwischen Individuen). Dort gibt es, außer eben der Variante von Foxp2, keine Differenz, auch nicht in der Geschwindigkeit der Evolution der Genome, unseres hat sich nicht rascher entwickelt als ihres.
Deshalb hat Joseph Call (MPI Evolutionäre Anthropologie, Leipzig) eine neue Suche eingeschlagen: Er hat das Metabolom analysiert, es gibt Auskunft darüber, was im Stoffwechsel der Zellen wie rasch umgesetzt wird, analysiert wurden verschiedene Gewebe von Mäusen, Makaken, Schimpansen und Menschen. Da zeigte sich eine Differenz: Die Metabolome von Mäusen, Makaken und Schimpansen sind so verschieden wie ihre Genome. Unseres aber zeigt starke Abweichungen vor allem in zwei Geweben, dem Gehirn und den Muskeln(PLoS Biology, 27.5.). Ersteres ist stark, letztere sind schwach, das hat Call in eher handgestrickten Zusatzexperimenten gezeigt: Jeder Schimpanse hat mehr Kraft in Armen und Beinen als durchtrainierte Sportler.
Es geht also um eine andere Verteilung der Energie, etwas Ähnliches kennt man schon, mit dem Wachstum unseres Gehirns ging eine Verkürzung des Darms einher. Nun muss nur noch geklärt werden, was hinter den Metabolomen steckt bzw. wie die Verteilung der Energie gesteuert wird.
Stronger Brains, Weaker Bodies.
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All animals do the same thing to the
food they eat — they break it down to extract fuel and building blocks
for growing new tissue. But the metabolism of one species may be
profoundly different from another’s. A sloth will generate just enough
energy to hang from a tree, for example, while some birds can convert
their food into a flight from Alaska to New Zealand.
For decades, scientists have wondered how our metabolism compares to that of other species. It’s been a hard question to tackle, because metabolism is complicated — something that anyone who’s stared at a textbook diagram knows all too well. As we break down our food, we produce thousands of small molecules, some of which we flush out of our bodies and some of which we depend on for our survival.
An international team of researchers has now carried out a detailed comparison of metabolism in humans and other mammals. As they report in the journal PLOS Biology, both our brains and our muscles turn out to be unusual, metabolically speaking. And it’s possible that their odd metabolism was part of what made us uniquely human.
When scientists first began to study metabolism, they could measure it only in simple ways. They might estimate how many calories an animal burned in a day, for example. If they were feeling particularly ambitious, they might try to estimate how many calories each organ in the animal’s body burned.
Those tactics were enough to reveal some striking things about metabolism. Compared with other animals, we humans have ravenous brains. Twenty percent of the calories we take in each day are consumed by our neurons as they send signals to one another.
Ten years ago, Philipp Khaitovich of the Max Planck Institute of Evolutionary Anthropology and his colleagues began to study human metabolism in a more detailed way. They started making a catalog of the many molecules produced as we break down food.
“We wanted to get as much data as possible, just to see what happened,” said Dr. Khaitovich.
To
do so, the scientists obtained brain, muscle and kidney tissues from
organ donors. They then extracted metabolic compounds like glucose from
the samples and measured their concentrations. All told, they measured
the levels of over 10,000 different molecules.
The scientists found that each tissue had a different metabolic fingerprint, with high levels of some molecules and low levels of others.
These distinctive fingerprints came as little surprise, since each tissue has a different job to carry out. Muscles need to burn energy to generate mechanical forces, for example, while kidney cells need to pull waste out of the bloodstream.
The scientists then carried out the same experiment on chimpanzees, monkeys and mice. They found that the metabolic fingerprint for a given tissue was usually very similar in closely related species. The same tissues in more distantly related species had fingerprints with less in common.
But the scientists found two exceptions to this pattern.
The first exception turned up in the front of the brain. This region, called the prefrontal cortex, is important for figuring out how to reach long-term goals. Dr. Khaitovich’s team found that the way the human prefrontal cortex uses energy is quite distinct from other species; other tissues had comparable metabolic fingerprints across species, and even in other regions of the brain, the scientists didn’t find such a drastic difference.
This
result fit in nicely with findings by other scientists that the human
prefrontal cortex expanded greatly over the past six million years of
our evolution. Its expansion accounts for much of the extra demand our
brains make for calories.
The evolution of our enormous prefrontal cortex also had a profound effect on our species. We use it for many of the tasks that only humans can perform, such as reflecting on ourselves, thinking about what others are thinking and planning for the future.
But the prefrontal cortex was not the only part of the human body that has experienced a great deal of metabolic evolution. Dr. Khaitovich and his colleagues found that the metabolic fingerprint of muscle is even more distinct in humans.
“Muscle was really off the charts,” Dr. Khaitovich said. “We didn’t expect to see that at all.”
It was possible that the peculiar metabolism in human muscle was just the result of our modern lifestyle — not an evolutionary shift in our species. Our high-calorie diet might change the way muscle cells generated energy. It was also possible that a sedentary lifestyle made muscles weaker, creating a smaller metabolic demand.
To test that possibility, Dr. Khaitovich compared the strength of humans to that of our closest relatives. They found that chimpanzees and monkeys are far stronger, for their weight, than even university basketball players or professional climbers.
The scientists also tested their findings by putting monkeys on a couch-potato regime for a month to see if their muscles acquired a human metabolic fingerprint.
They barely changed.
Dr. Khaitovich suspects that the metabolic fingerprint of our muscles represents a genuine evolutionary change in our species.
Karen Isler and Carel van Schaik of the University of Zurich have argued that the gradual changes in human brains and muscles were intimately linked. To fuel a big brain, our ancestors had to sacrifice other tissues, including muscles.
Dr. Isler said that the new research fit their hypothesis nicely. “It looks quite convincing,” she said.
Daniel E. Lieberman, a professor of human evolutionary biology
at Harvard, said he found Dr. Khaitovich’s study “very cool,” but
didn’t think the results meant that brain growth came at the cost of
strength. Instead, he suggested, our ancestors evolved muscles adapted
for a new activity: long-distance walking and running.
“We have traded strength for endurance,” he said. And that endurance allowed our ancestors to gather more food, which could then fuel bigger brains.
“It may be that the human brain is bigger not in spite of brawn but rather because of brawn, albeit a very different kind,” he said.
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