Campfires, Explained解密营火

布莱恩·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].”

我们的祖先用火时，衣物和农业尚未面世，连车轮都还没发明出来。

营火由来已久，本文以科学为根据，对其加以阐释，要点如下：营火对人类进化的重要性;营火燃烧时的化学原理;此种古老燃料对人体健康与生存环境的影响。

营火对人类进化意义非凡

在《生火》一书中，生物人类学家理查德·兰厄姆提出，有了营火，人类开始烹制肉类，食用熟肉，远古人类的脑容量由此得以扩大。他写道：“最先烹制肉类的人从中得到更多能量，故而具有生物学优势。他们生存下来，生育质量胜过以往，基因由此传播……人体结构、生理机能、生态、生命史、人类心理和人类社会都由此改观。”

在早期阶段，我们的祖先或许并不知道如何生火。他们只懂得如何在电闪雷鸣或灌木自燃后，将火种保存下来。在《进化心理学》期刊上，人类学家克里斯托弗·达纳·林恩提出，由于不会生火，人群需要合作，以便取火并保留火种。如此持续合作，迫使人类形成社会包容的认知，将他人视为后续的合作伙伴。

火还催生了钢铁厂与蒸汽机的发明，人类由此可以真正随心所欲地改造世界。

火，究竟为何物？

你眼中的火焰，其实是复杂化学反应的产物，学名叫作“高温分解”。所以你会看到：木料转化为可燃气，热量将气体点燃，火光则来自激发态电子的活动。

也可以这样理解：整个燃烧过程，相当于把木料无限撕碎。撕裂时释放化学键，将能量消解为光和热。

不过，凡是设法用一根火柴点燃整块原木的人都知道，要想把火点着，得费天大的力气。仅凭一根火柴，或者钢片与燧石擦出的火花，不可能点着原木。

你得消耗一点能量，将其转化为自持反应。木材各组分必须吸收足够热量，才能开始高温分解。

原理如下：植物纤维升温后，组织（主要由一種叫作纤维素的分子生成）开始降解及分解。 植物组织不断升温，将水分逼出细胞，然后细胞分裂，形成挥发性易燃气体。

以上过程均需氧气参与，因为燃烧属于氧化反应。

可燃气起火，原木进一步分解。 实际上，这种气体含有数百种碳基化合物。 其中一些形成烟灰，在火焰中继续分解。 如果火焰完全燃烧，原木中的大分子会分解为二氧化碳和水蒸气。

但是，这一过程为何产生光？

光来自电子，这种电子释放额外能量，从一种激发态转移到较低能级的激发态。（你知道金属赤热时如何发光吗？木材燃烧时产生的火光与此同理，只是吸收热量的并非金属，而是烟尘微粒。）

营火烟尘是污染物吗？

营火烟尘中的微粒也会导致雾霾和烟霾。例如，在明尼苏达州，户外休闲篝火广受欢迎，而空气中约5%的微粒都来自此种烟尘。

就二氧化碳排放而言，木材燃烧后，如果种下新树，则可中和营火烟尘排出的碳。“但这不能一概而论。”明尼苏达州污染控制机构的环境研究科学家莉萨·赫施贝格尔表示：“（评估碳排放，）关键是了解该木材的生长和运输情况。只有了解它的整个生命周期，才能知道自己（在碳排放方面）是领先还是落后。”

（译者为“《英语世界》杯”翻译大赛获奖选手）