雅思课外读物--Why people fall in love

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爱由情生;情始于脑。一切灵长类皆有脑，独独人类进化出了“爱情”，何也?或曰“爱情”源于避免弑婴，或曰“爱情”脱胎于母爱。无论如何，人之为人，就是因了这令人疯狂的”爱情“玩意儿。

Your heart beats a little faster, glands(腺)open to secret tiny dribbles of

sweat, and your body starts producing hormones, which make you feel a bit

giddy(晕眩的)and warm inside.

These are some of the biological processes that occur as you are thrust into

the early throes(痛苦，挣扎)of love – or infatuation(迷恋), it can be hard to tell

which it is.

Love is such a pervasive(普遍的)part of our humanity that art and culture is

filled with references to love won and love lost. Libraries have shelves of

books filled with romantic prose. "Love is not time's fool," wrote Shakespeare

in sonnet 116: "Love alters not with his brief hours and weeks / But bears it

out even to the edge of doom."

It seems Shakespeare was more correct than he could have known. Peer into the

evolution of love in the animal kingdom and it becomes apparent that love had

its beginnings long before the advent(来临)of humanity. What's more, it could have

been born out of something quite sinister(险恶的;不祥的).

The journey to love as we know it today began with sex, which was one of the

first things life on Earth figured out how to do. Sex began as a way to pass on

an organism's genes to the next generation.

To love, life first needed a brain that could deal with emotions. It was not

until a few billion years after life began that the brain began its journey to

existence. At first it was only a small clump of cells.

Fast forward to around 60 million years ago, when the first members of our

family, the primates(灵长目动物), appeared. Over millions more years of evolution,

some primates would evolve ever bigger brains, eventually producing modern

humans.

But there was a problem. As our brains grew, our babies had to be born

earlier in development. Otherwise their heads would be too big to pass through

the birth canal.

As a result, baby gorillas, chimps and humans are almost entirely helpless.

Their parents therefore had to spend ever more time caring for them.

This prolonged childhood created a new risk.

In many primates today, a mother with a dependent infant is unavailable to

mate until her infant is weaned(断奶). To get access to her, a male would first

have to kill her child. This sort of targeted infanticide(弑婴)goes on in many

species, including gorillas, monkeys and dolphins.

This led Kit Opie of University College London in the UK and his colleagues,

to propose a startling idea. Almost a third of primates form

monogamous(一夫一妻的)male-female relationships, and in 2013 Opie suggested that this

behaviour had evolved to prevent infanticide.

His team peered back into the family tree of primates to reconstruct how

behaviours like mating and parenting changed over the course of evolution. Their

analysis suggested that infanticide has been the driving force for monogamy for

20 million years, because it consistently preceded(先于......)monogamy in

evolution.

Other species found different solutions, which is why not all primates are

monogamous. For instance, chimps and bonobos minimise the risk of infanticide by

being highly promiscuous(混交的). The males do not kill babies because they do not

know which are theirs.

But in those species where males and females started bonding strongly, their

offspring's chances of survival improved because the males could help out with

parenting. As a result, monogamy was favoured by evolution, says Opie.

This process may have been a one-way street, says Robin Dunbar of the

University of Oxford in the UK. It could have resulted in major changes in the

brain, "to keep the pair-bond together for life". This includes a preference for

your partner and antagonism(敌对)towards potential rivals.

This in turn could have been the "kick" that changed human evolution, says

Opie. Extra male care helped early human societies grow and thrive(兴旺), which in

turn "allowed our brains to grow larger than our closest relatives".

There is evidence to back this up. As brain size started to expand, so did

cooperation and group size. We can see a trend towards larger groups and more

cooperation in the early-human species Homo erectus, which lived almost two

million years ago.

What's more, it seems that aspects of love depend on regions of the brain

that only appeared quite recently in our evolutionary history.

Stephanie Cacioppo of the University of Chicago in Illinois, US, scoured the

scientific literature to find fMRI brain imaging studies that examined the parts

of the brain involved in love. She found that the most intense and "abstract"

states of love rely on a part of the brain called the angular gyrus(角形脑回).

This is known to be important for certain aspects of language, like

metaphors. This makes some sense, as without complex language we cannot express

the more refined and intense aspects of our emotions. Conceivably, Shakespeare's

angular gyrus was active when he penned his love sonnets.

The angular gyrus is only found in great apes and humans.

We do not actually know what role it plays in apes' emotions, says Cacioppo,

because "complementary fMRI experiments have not been performed on apes". So we

do not know what chimpanzees feel about their mates. Obviously they do not write

sonnets, but neither do most humans.

Still, Cacioppo's findings offer some support to the idea that our growing

brains helped love to flourish(兴旺).

However, Opie's idea that infanticide kick-started this process is

controversial. Not everyone agrees it played any role in the development of

monogamy.

Anthropologist Robert Sussman of Washington University in St Louis in

Missouri, US is one of the sceptics(怀疑论者). He says that both monogamy and

infanticide are such unusual behaviours that they are unlikely to be linked.

There are alternatives. A 2014 study suggested monogamy evolved as an

outgrowth of a "mate guarding strategy": that is, males staying with a female to

ensure that no one else mates with her.

One year later, another study reconstructed the evolution of another group of

primates called lemurs(狐猴). It found that female competition could have

encouraged pair bonds.

Opie disagrees. He says the methods in these studies "cannot be used to

determine the switch to monogamy".

What is certainly true is that many primates get by just fine without

pair-bonded parents, and presumably without anything akin to(与.....类似)romantic

love. But there is one thing all primates do have in common: a strong

mother-child bond.

This is true, "even in the nocturnal(夜晚出行的)primates that live solitarily,"

says Sussman. He suggests that the brain processes underlying the mother-child

bond were "hijacked" to create romantic

love.(他提出，支配母亲-孩子关系的大脑加工过程被“绑架”来创造了浪漫爱情。)

There is evidence from neuroscience to suggest he is right.

Love is hard to define, but neuroscientists agree that there are several

overlapping(交叉的) stages.

The first stage is sexual desire: we feel attracted to another person.

Touching them releases feel-good chemicals and we experience an intense longing

to be with them.

Parts of our limbic system(大脑边缘系统), one of the more ancient bits of the human

brain, are active during this stage. This includes the insula(脑岛), an area known

to be involved in intense emotional experiences. The ventral striatum is also in

overdrive. It is the hub of the brain's reward system, and when we see an

attractive face it lights up: we are rewarded simply by looking at the person we

desire.

As desire moves onto the next stage – romantic love – the limbic system again

plays a key role. It pumps out the feel-good chemical dopamine(多巴胺)and the

hormone oxytocin(催产素), which binds people together.

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