A toxic chemical
As anyone who's had too much knows, alcohol – called ethanol by scientists – can be a toxic chemical. And humans' ability to tolerate drinking relies on one enzyme, called ADH4, the researchers explained.
The new study suggests that, beginning about 10 million years ago, humans' primate ancestors developed a gene mutation to produce ADH4. In times when food was scarce, this enabled them to safely consume a noxious byproduct of rotting fruit – alcohol.
"One hypothesis has been that our ancestors – ancestors of humans that we share with gorillas and chimps – may have been exposed to ethanol millions of years ago as a result of a long history as frugivores [fruit eaters]," explained study lead author Matthew Carrigan. Carrigan is an assistant professor in the department of natural sciences at Santa Fe College in Gainesville, Florida. "Because fruit can naturally ferment, we may have adapted naturally to the consumption of these foods."
The researchers noted that the ADH4 class of proteins is found in the upper digestive tracts of many vertebrates, including humans, horses, mice and rats.
Resurrected enzymes
In their study, published online in the Proceedings of the National Academy of Sciences, Carrigan's team worked with paleogeneticists, researchers who study ancient proteins found in now-extinct animals and species.
Together, they resurrected "enzymes from different points in our ancestors' history," Carrigan explained, looking at samples of ancient ADH4 drawn from key points over 70 million years of primate evolution.
"The evidence suggests that it was actually about 10 million years ago that our ancestors' enzymes changed as a result of exposure to ethanol, which means it's a very long-standing exposure," Carrigan said.
The finding might change the way scientists look at both alcohol and human evolution, he added.
"The same time our adaptation to ethanol took place, our ancestors also adapted to life on the ground," Carrigan explained. "Prior to this our ancestors were living almost exclusively in the trees, rarely coming to the ground, which means they weren't eating fruit off the ground. But once they did, they were being exposed to fruit which is older than fruit found in the tree – and more likely to be fermented," he said.
New insights into alcoholism
"So my suspicion is that our ancestors who developed this mutation were using fermented fruit found on the ground as a 'fallback food' – it probably wasn't their first choice. But if times got tough, if the climate changed perhaps, and forced our ancestors to adapt to life on the ground, this might be the driver behind their adaptation to ethanol," said Carrigan.
"I should say that at this point this is all speculation," he added. "But basically it suggests that we adapted to ethanol simply so we could tolerate it."
The findings might even offer new insights into alcoholism, Carrigan suggested.
"If it turns out to be the case that ethanol has been a beneficial and important part of our diet for some time, then the question is how does this understanding affect our thinking about alcoholism?" he said.
How a particular person's genes deal with alcohol might give clues to alcoholism prevention and treatment, Carrigan believes.
"It's important to appreciate that there is natural amount of diversity in [individuals'] genes," he explained.
"And understanding these differences – and how these differences might affect a person's risk for addiction – could be really important when trying to understand how people interact with ethanol differently."
How different races handle alcohol
It is evident that not all races are created equally, at least on a physical level. The Olympic Games provide ample evidence that Africans are generally better at running and Europeans better at swimming.
It might not be politically very correct or sensitive to remark on these things, but there is a lot of anecdotal evidence that certain racial groups are better at handling alcohol than others.
It seems that Native Americans, Australian Aborigines and the African Khoisan tend to get hooked on alcohol more easily than Europeans. (Turning these people into alcoholics was of course a very convenient way for their colonial masters to break their spirit and enslave them.)
Asians also don't have a reputation for holding their alcohol very well and, generally speaking, Europeans can drink their Chinese and Japanese counterparts under the table without too much trouble.
Native Americans and Aboriginals may become alcoholics more easily because their bodies tend to break down ethanol into water and acid more slowly than other races.
High levels of ADH
When we drink alcohol, the ethanol is metabolised into acetaldehyde by an enzyme called alcohol dehydrogenase (ADH), and the more ADH our bodies produce, the quicker the process. Based on the analysis of chromosomes 4 and 11, many Native Americans lack this enzyme which increases their risk of becoming alcoholic.
Surprisingly the Chinese and Japanese produce a lot of ADH and 85% of their population produce very high levels of this enzyme. Caucasians and Africans produce less, and Native Americans and Asian Indians produce hardly any.
However, this doesn’t mean that Asians are great drinkers. This is because the acetaldehyde that was created by the first step hasn’t been turned into acetic acid and water yet. These are created by another enzyme called acetaldehyde dehydrogenase 2 (ALDH 2) – and about half of Chinese and Japanese lack the normal amount of this enzyme.
Because of this, acetaldehyde builds up very quickly when Asians drink alcohol. (Firstly of all because of the high amount of ADH, and secondly because of the lack of ALDH 2.) It's unfortunately for them, because acetaldehyde makes you a lot sicker than ethanol itself. This disadvantage is why a lot of Asians have trouble holding their liquor. A sign of high acetaldehyde levels is a bright red face.
These are just generalisations, but genetic differences between races cannot be ignored. One explanation for low or non-existent levels of ADH is that some Aboriginal peoples were not exposed to alcohol before the Europeans arrived. It is however only an assumption and a lot more research on alcohol metabolism needs to be done.
Source: Health24
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