Some like it hot: Site of human evolution was scorching

Jun 08, 2010

If you think summer in your hometown is hot, consider it fortunate that you don't live in the Turkana Basin of Kenya, where the average daily temperature has reached the mid-90s or higher, year-round, for the past 4 million years.

The need to stay cool in that cradle of human evolution may relate, at least in part, to why pre-humans learned to walk upright, lost the fur that covered the bodies of their predecessors and became able to sweat more, Johns Hopkins University earth scientist Benjamin Passey said.

"The 'take home' message of our study," said Passey, whose report appears this week in the online early edition of , "is that this region, which is one of the key places where fossils have been found documenting human evolution, has been a really hot place for a really long time, even during the period between 3 million years ago and now when the ice ages began and the became cooler."

Passey, an assistant professor in the Morton K. Blaustein Department of Earth and Planetary Sciences at the university's Zanvyl Krieger School of Arts and Sciences, says that conclusion lends support to the so-called "thermal hypothesis" of human evolution.

That hypothesis states that our pre-human ancestors gained an in walking upright because doing so was cooler (when it is sunny, the near-surface air is warmer than air a few feet above the ground) and exposed their body mass to less sunlight than did crawling on all fours. The loss of body hair (fur) and the ability to regulate body temperature through perspiration would have been other adaptations helpful for living in a warm climate, according to the hypothesis.

"In order to figure out if (the thermal hypothesis) is possibly true or not, we have to know whether it was actually hot when and where these beings were evolving," he said. "If it was hot, then that hypothesis is credible. If it was not, then we can throw out the hypothesis."

Evaluating whether the ancient Turkana Basin climate was, in fact, the same scorching place it is today has been difficult up until now because there are very few direct ways of determining ancient temperature. Efforts to get a handle on temperatures 4 million years ago through analysis of fossil pollen, wood and mammals were only somewhat successful, as they reveal more about plants and rainfall and less about temperature, Passey said.

Passey, however, previously was part of a team at the California Institute of Technology that developed a geochemical approach to the "temperature problem." The method involves determining the temperatures of carbonate minerals that form naturally in soil (including a sedimentary rock called "caliche" and hard pan, which is a dense layer of soil, usually found below the uppermost topsoil layer) by examining "clumps" of rare isotopes. (Isotopes are atoms of the same element that have different masses due to differences in the number of neutrons they contain.)

In the case of soil carbonates common in the Turkana Basin, the amount of rare carbon-13 bonded directly to rare oxygen-18 provides a record of the temperature during the initial formation of the mineral. It told the team that soil carbonates there formed at average soil temperatures between 86 and 95 degrees Fahrenheit, leading to the conclusion that average daytime air temperatures were even higher. In other words, it was hot way back then in what is now northeastern Kenya.

"We already have evidence that habitats in ancient East Africa were becoming more open, which is also hypothetically part of the scenario for the development of bipedalism and other , but now we have evidence that it was hot," Passey said. "Thus, we can say that the 'thermal hypothesis' is credible."

Explore further: NASA ocean data shows 'climate dance' of plankton

More information: PNAS paper: "High-temperature environments of human evolution in East Africa based on bond ordering in paleosol carbonates," by John Eiler et al.

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NotAsleep
4.3 / 5 (3) Jun 08, 2010
It would seem that every other hairy animal in Kenya would prove that losing hair certainly isn't evolutionarily necessary. Apes, gorillas, etc. also show that bipedal movement isn't a solid factor.

In the desert, full body garments are usually worn to prevent heat stroke. It would seem that hair would give a distinct ADVANTAGE to people living in a warm, sunny climate by acting as a heat sink while preventing sunlight from reaching the skin. In the quest to prove hair-loss as an evolutionary advantage, I suspect "hot weather" probably shouldn't play a part
Caliban
3.7 / 5 (3) Jun 08, 2010
Not to mention that there might be some error involved in naming the Lake Turkana area the origin of Hominidkind. Afterall- hot'n'dry are the best conditions for preserving skeletal remains, and hence favor later fossilization, whereas a wetter environment is less favorable of the process, but seems more likely as a plentiful source of food for the protomen. Perhaps the greater number of fossils is not related to "origin" at all.
RobertKarlStonjek
5 / 5 (5) Jun 08, 2010
"...an evolutionary advantage in walking upright because doing so was cooler ..."
The *really* cool early humans danced...
PinkElephant
4.7 / 5 (3) Jun 08, 2010
@NotAsleep,
It would seem that every other hairy animal in Kenya would prove that losing hair certainly isn't evolutionarily necessary.
Then again, not every other hairy animal in Kenya dwelled in the past or dwells today in the Turkana Basin.
Apes, gorillas, etc. also show that bipedal movement isn't a solid factor.
Ditto.
It would seem that hair would give a distinct ADVANTAGE to people living in a warm, sunny climate by acting as a heat sink
Hair traps air next to the skin, thereby acting as an insulator, not a heat sink, and preventing the body from shedding metabolic heat.

@Caliban, good points.
NotAsleep
5 / 5 (1) Jun 09, 2010
@ PinkElephant

Whoops, my bad on hair as a heat sink... I was thinking about polar bears when I wrote that, which produces the absolute opposite effect of what I wrote...

In regards to your other comments, the fact remains that there are very few mammals as hairless as humans. The article makes a valid point in creating a comparison between hair loss and ability to sweat, but even this comparison becomes difficult considering men (the primary hunters in the olden days) are hairier than women. Also complicating that is the fact that horses are also efficient sweaters.

I'm certainly not trying to refute the article, heat dispersion probably drove us away from being hairy, but perhaps stone-age men just had the same disposition to hairy stone-age women that we have today...
LuckyBrandon
2 / 5 (2) Jun 09, 2010
NotAsleep-being hunters, then men would have moved around more than the women, so air would be moving against them to help cool them (regardless of whether it was cool or hot air)...whereas the females would likely have been "sitting in one place" taking care of the children, which would cause them to absorb more heat...
Just thinking out loud, but seems logical....
NotAsleep
5 / 5 (1) Jun 09, 2010
Try a control test on yourself. Sit around for twenty minutes. After that, run for twenty minutes. Do you feel hotter running or sitting around?
PinkElephant
4.5 / 5 (2) Jun 09, 2010
@NotAsleep,
men (the primary hunters in the olden days) are hairier than women
Depends on which men. If you look at men of tribes living in equatorial Africa, I think you'll find they tend to be so hairless, it's downright embarrassing.
Also complicating that is the fact that horses are also efficient sweaters.
That's true, but they're also much more efficient walkers and runners, and they're adapted to more varied ambient conditions: e.g. for a horse, it's easy to shrug off a minor frost spell, whereas a human without clothes would die of hypothermia.
I'm certainly not trying to refute the article
And I'm not saying the premise of the article is necessarily true; there are competing hypotheses. But if you're going to try to refute something, you'll have to make more forceful counter-arguments :-)
tj10
1.7 / 5 (3) Jun 10, 2010
It is interesting how we come up with stories to try and explain the differences between humans and animals. Lack of hair is just one of those traits. The "just-so" story is not scientific in that it cannot be tested. Anyone can make up a story as to what might have happened and the press regurgitates it often times as fact unless you really read all the details.

If what they say is true, then why didn't humans living in cold parts of the world keep their hair? Or if they lost it once, certainly re-evolving it would be a cinch.

The other fact that makes this story questionable has already been pointed out by other posters: Why didn't the mammals living in same region or other hot regions of the world lose their hair too if it was such a great evolutionary advantage? You would think it would have helped out these 4 legged folk even more since they are closer to the ground.

This article is impossible to refute since it cannot be proven to start with. It's just 1 possible guess.
PinkElephant
5 / 5 (2) Jun 10, 2010
If what they say is true, then why didn't humans living in cold parts of the world keep their hair? Or if they lost it once, certainly re-evolving it would be a cinch.
First of all, have you SEEN some of the northern-folk? As hairy as carpet. I can personally testify to that (unfortunately.) There's probably more than a bit of Neanderthal in them/me. (Though it's still not enough fur to actually keep one warm in winter.)

Secondly, by the time /modern/ humans left Africa, and perhaps much earlier, they almost certainly had the technology to make clothes out of animal skins and/or certain plants. Clothes are much better than hair.

Third, it's not that easy to "re-evolve" something that's been lost. Mutations are not need-based; they're accidental. Even absent clothing, the odds that a fur-inducing mutation would fortuitously ensue among humans that were trying to colonize northern climes, within just a couple of dozen millennia after leaving Africa, are rather low.
LuckyBrandon
2.3 / 5 (3) Jun 11, 2010
notasleep-it depends on where you're sitting...if you are sitting out in the blazing sun, then sitting still will easily be hotter. When hunting, you aren't typically actually running...maybe walking, but predominantly hiding in an area where you aren't visible (aka, out of the sun)...and if hunting in woodland areas, there is shade over you...
But I do understand your point...
GaryB
not rated yet Jun 12, 2010
Not to mention that there might be some error involved in naming the Lake Turkana area the origin of Hominidkind.


My pet peev too. Maybe we evolved in the rainy moist areas but everything rotted fast there. I think a lot of ancient study might be contaminated by a huge sample bias that must have existed: some places just preserve things better than others. It is also possible that we are hairless because its harder for parasites to take hold, or it was just a random mutation that came along with our brain size and its more advantageous to be naked under your cloths.
Bog_Mire
1 / 5 (1) Jun 17, 2010
my theory: our skulls became so big to accommodate our bulging brains that childbirth became dangerous to mums, so we shed our hair to help her out...smooth things slide out easier. And if we get any smarter the human race will be exclusively cesarean section born or we die out. or test tubes. or matrix style ...umm... things