Campfires, Explained解密營火

布萊恩·雷斯尼克

Before the invention of clothing， agriculture， and even the wheel， our ancestors were playing with fire.

Here’s a science-backed1 guide to the ancient practice of building a campfire2， from its importance for human evolution to the chemistry of how it burns to this age-old3 fuel’s impact on our health and our environment.

Campfire was monumentally4 important to our evolution

In the book Catching Fire， biological anthropologist5 Richard Wrangham argues that campfires—and the subsequent6 invention of cooking meat and eating it—were the catalyst7 that allowed our ancestors to develop big brains. “The extra energy [in the cooked meat] gave the first cooks biological advantages，” Wrangham writes. “They survived and reproduced better than before. Their genes spread. … There were changes in anatomy8， physiology9， ecology， life history， psychology， and society.”

In these early days， it’s likely our ancestors didn’t actually know how to start fires. They only knew how to maintain them—after a lightning strike or spontaneous conflagration10 of brush got one started. Anthropologist Christopher Dana Lynn writes in the journal Evolutionary Psychology that inability to start fires would have required groups to coordinate activities to access and maintain them. This continual cooperation would have put pressure on cognitive capacities for social tolerance11， conceiving of others as collaborators in future cooperation

Fire would also lead to inventions like the steel mill and the steam engine， which would allow humans to literally reshape the world to their likings12.

So what， exactly， is fire？

When you look at flames， you are seeing the results of a complex chemical reaction called pyrolysis13. You’re seeing wood turned into gas， gas ignited14 by heat， and light from the excitement of electrons.

Here’s another way to think about it： The entire process of a fire is about tearing a log into as many pieces as possible. The tearing releases chemical bonds15， expending energy as heat and light.

But anyone who has tried to ignite a whole log with just a single match knows that it takes a lot to get a fire going. You can’t do it with a single match or spark from a piece of steel on flint16.

You have to take a tiny bit of energy and transform it into a self-sustaining reaction17. Each component of the wood has to absorb enough heat to begin the pyrolysis process.

Here’s how it goes： As plant fibers heat up， the plant’s tissues—mostly made out of a molecule18 called cellulose19—start degrade and break down. As the tissue gets hotter and hotter， water is driven out of the cells， and they then break apart， forming volatile20， combustible21 gases.

All of this needs to be done in the presence of oxygen， as fire is an oxidation reaction22.

The ignition of the gas continues the process of breaking down that log further and further. Inside that gas are actually hundreds of carbon-based compounds23. Some of these form soot24 and then are broken down further in the flame. If a fire burns perfectly， the log will break down all the big molecules into carbon dioxide and water vapor.

But why does this process create light？

It comes from the electrons releasing extra energy—going from an exited state to a less excited state25. （You know how metal glows when it’s heated red-hot？ The same thing is happening in the flame， but instead of metal， it’s the tiny particles of soot absorbing the energy.）

Is wood smoke a pollutant？

The particles from wood smoke also can contribute to smog and haze. In Minnesota， for instance， where recreational outdoor fires are popular， recreational wood smoke accounts for around 5 percent of all the fine particles26 released to the air.

In terms of carbon dioxide emissions， wood smoke can be carbon neutral if the wood you burn is replaced by new growth. “But it’s not a slam dunk27，” Lisa Herschberger， an environmental research scientist with Minnesota’s pollution control agency， says. “It will be really important [for emissions] to learn how that wood was grown， how it was transported. It takes knowing the whole life cycle of the wood to know if you’re ahead or behind [on carbon emissions].”

我們的祖先用火時，衣物和農業尚未面世，連車輪都還沒發明出來。

營火由來已久，本文以科學為根據，對其加以闡釋，要點如下：營火對人類進化的重要性;營火燃燒時的化學原理;此種古老燃料對人體健康與生存環境的影響。

營火對人類進化意義非凡

在《生火》一書中，生物人類學家理查德·蘭厄姆提出，有了營火，人類開始烹制肉類，食用熟肉，遠古人類的腦容量由此得以擴大。他寫道：“最先烹制肉類的人從中得到更多能量，故而具有生物學優勢。他們生存下來，生育質量勝過以往，基因由此傳播……人體結構、生理機能、生態、生命史、人類心理和人類社會都由此改觀。”

在早期階段，我們的祖先或許并不知道如何生火。他們只懂得如何在電閃雷鳴或灌木自燃后，將火種保存下來。在《進化心理學》期刊上，人類學家克里斯托弗·達納·林恩提出，由于不會生火，人群需要合作，以便取火并保留火種。如此持續合作，迫使人類形成社會包容的認知，將他人視為后續的合作伙伴。

火還催生了鋼鐵廠與蒸汽機的發明，人類由此可以真正隨心所欲地改造世界。

火，究竟為何物？

你眼中的火焰，其實是復雜化學反應的產物，學名叫作“高溫分解”。所以你會看到：木料轉化為可燃氣，熱量將氣體點燃，火光則來自激發態電子的活動。

也可以這樣理解：整個燃燒過程，相當于把木料無限撕碎。撕裂時釋放化學鍵，將能量消解為光和熱。

不過，凡是設法用一根火柴點燃整塊原木的人都知道，要想把火點著，得費天大的力氣。僅憑一根火柴，或者鋼片與燧石擦出的火花，不可能點著原木。

你得消耗一點能量，將其轉化為自持反應。木材各組分必須吸收足夠熱量，才能開始高溫分解。

原理如下：植物纖維升溫后，組織（主要由一種叫作纖維素的分子生成）開始降解及分解。 植物組織不斷升溫，將水分逼出細胞，然后細胞分裂，形成揮發性易燃氣體。

以上過程均需氧氣參與，因為燃燒屬于氧化反應。

可燃氣起火，原木進一步分解。 實際上，這種氣體含有數百種碳基化合物。 其中一些形成煙灰，在火焰中繼續分解。 如果火焰完全燃燒，原木中的大分子會分解為二氧化碳和水蒸氣。

但是，這一過程為何產生光？

光來自電子，這種電子釋放額外能量，從一種激發態轉移到較低能級的激發態。（你知道金屬赤熱時如何發光嗎？木材燃燒時產生的火光與此同理，只是吸收熱量的并非金屬，而是煙塵微粒。）

營火煙塵是污染物嗎？

營火煙塵中的微粒也會導致霧霾和煙霾。例如，在明尼蘇達州，戶外休閑篝火廣受歡迎，而空氣中約5%的微粒都來自此種煙塵。

就二氧化碳排放而言，木材燃燒后，如果種下新樹，則可中和營火煙塵排出的碳。“但這不能一概而論。”明尼蘇達州污染控制機構的環境研究科學家莉薩·赫施貝格爾表示：“（評估碳排放，）關鍵是了解該木材的生長和運輸情況。只有了解它的整個生命周期，才能知道自己（在碳排放方面）是領先還是落后。”

（譯者為“《英語世界》杯”翻譯大賽獲獎選手）