Authors: Carl Zimmer
A few other researchers have also found tantalizing hints of the hereditary power of RNA in animals.
Antony Jose, a biologist at the University of Maryland, tracks RNA molecules produced inside the body of a tiny worm called
RNA molecules created in the worm's brain can make their way across its body and end up inside its sperm, where it
turns off a gene. Other researchers have found that RNA molecules in the worms can turn off the same gene in the next generation, and for several generations after that. It appears that the RNA molecules sustain themselves through the generations by
spurring young worms to make more copies of themselves.
We are not worms, of course, but a number of experiments have demonstrated that human cells can send RNA molecules to each other on a regular basis. Very often, they are delivered in
tiny bubbles, called exosomes. Scientists have observed more and more types of cells releasing exosomes, and more and more taking them up. In some species,
embryos may use exosomes to send signals between parts of the body to make sure they all develop in sync.
Heart cells may release them after a heart attack to trigger the organ to repair itself. Cancer cells spew out exosomes with exceptional abandonâprobably as a way to manipulate surrounding healthy cells into becoming their servants. In 2014, an Italian biologist named
Cristina Cossetti observed that exosomes cast off by cancer cells in male mice could deliver their RNA into their sperm cells.
These studies are far from conclusive, but they've been provocative enough to send scientists back to reread
The Variation of Animals and Plants Under Domestication.
Darwin's gemmules certainly don't gather genes from around the body. But perhapsâjust perhapsâexosomes are a modern incarnation of gemmules, ferrying the RNA molecules that allow the experiences of one generation to influence the next.
But even if there is a link from somatic cells to the germ line and to future generations, it won't be enough to resurrect Lamarck. What made Lamarck's theory so seductive in the nineteenth century was the idea that the acquired traits were
. In other words, they helped animals and plants survive, enabling species to fit themselves to their environment. Lamarck believed his version of evolution could explain why species were so well matched to their surroundings. In Lamarck's world, giraffes stretched their necks and ended up with the longer necks they needed to get food.
There is no solid evidence that transgenerational epigenetic inheritance is adaptive in the sense Lamarck intended. The few experiments that come closest to support have been carried out on plants. In one such study,
researchers at Cornell University put caterpillars on a small flowering plant called
. The plants responded by making toxic chemicals that slowed down the onslaught. The researchers then bred the plants for two generations and then unleashed a new assault of caterpillars on the third-generation offspring. The plants still made high levels of toxins that stunted the growth of the insects.
Martienssen finds these experiments intriguing but doesn't see them as solid proof of Lamarckism.
is the lab rat of the plant kingdom, for example, having been bred by scientists for many generations in caterpillar-free conditions. Their response to insect enemies may not reflect what happens outside, in the insect-infested world.
“Finding that is still a Holy Grail of epigenetics,” Martienssen said. “I mean, there are reports out there, but nothing has really, really stuck.”
It's entirely possible that some inherited epigenetic changes are good for plants. But it's also possible that others are bad, and still others indifferent. The flowers of
grabbed Linnaeus's attention, but no one has demonstrated that they do the plants any more good than the ordinary toadflax flower. An epigenetic flip simply swapped one flower for another.
Dandelions, scientists have found, can inherit epigenetic patterns that make them sprout early or late. Wild populations of
inherit some patterns that make some of their roots grow deep and others shallow. It's possibleâalthough it has yet to be provenâthat this overall variety helps out plants. If a drought strikes a prairie, a population of flowers may avoid extinction because of the long roots that a few of them have, thanks to the lingering luck of the epigenetic draw.
Regardless of what transgenerational epigenetic inheritance does for plants, Martienssen told me, he saw them as a legitimate part of how their ancestors influence their descendants.
At one point in our conversation, Martienssen surprised me by asking if I had ever heard of Luther Burbank. I had indeed; just a few weeks before,
I had made my own pilgrimage to his garden in Santa Rosa. But here at Cold Spring Harbor, at a modern shrine to genetics, I didn't expect to hear his name. Martienssen said that Burbank fascinated him. Burbank might not have been a rigorous scientist, but he could perceive patterns that still matter to science today. Martienssen stared off and recited to me a line of Burbank's that he drops whenever he can into his lectures and papers.
“Heredity,” Burbank declared, “
is only the sum of all past environment.”
I picked up
my older daughter, Charlotte, from preschool. As I was pulling her lunch bag and coat from her cubby, I came across a typed sheet of paper. I stopped for a moment to read it. It was from a Yale graduate student who was studying how children learn. He was looking for parents who would volunteer their young.
I got home and immediately e-mailed the student, a young man named Derek Lyons, and asked what he was investigating. Lyons responded later in the day. He explained that his research might give him some clues about what makes the human mind unique, perhaps even about how our species evolved.
I suppose a lot of parents might have been put off by that. But I signed right up.
Lyons later paid a visit to Charlotte's preschool to give her a screening test, and a few days later I drove her to New Haven for more trials. I held her hand as we shuffled down the worn steps in Sheffield-Sterling-Strathcona Hall to reach a basement lab. Lyons, thin, whippet-faced, greeted her like an old friend. Charlotte smiled back and took his hand, following him into another room. I read a book while the experiment took place. When the two of them returned, Charlotte was carrying little plastic animals.
I asked Charlotte how it went as we left the building. She shrugged and
said it was fun. We had been talking a lot about princesses and atoms on the ride over, and she was more interested in getting back to those subjects as we drove home. Whatever secrets she had helped unlock would have to remain locked for now.
Lyons was studying young children like Charlotte to follow up on a series of earlier experiments on young chimpanzees. A team of Scottish scientists had shown each chimpanzee a clear plastic box with some fruit inside. The box had an assortment of doors and bolts on it. The scientists demonstrated to the apes how to slide open the bolts and open the doors in order to get to the reward. Sometimes they showed the chimpanzees how to open the box using the minimum number of actions. Other times, they added needless flourishes, like opening doors and shutting them, or using a bolt to tap the sides of the box.
The chimpanzees would patiently watch the demonstration, and then seize the box as soon as they got the chance. They would open it as swiftly as possible, jettisoning the needless extra steps, and grab the fruit. Their own sense of physics trumped any urge they might have to imitate humans.
Lyons's experiments were designed to observe how the battle of physics versus imitation plays out in the mind of a child. A couple of weeks after Charlotte's trial, he invited me back to his lab to take a look at the videos he had filmed. Driving into New Haven, I couldn't help feeling as if she had taken some sort of interspecies SAT test. I hoped she had scored one for
We sat down in front of Lyons's computer monitor, and he switched on a video. I could see Charlotte sitting on a carpet at school, her legs folded to one side. She looked at Lyons as he told her he was going to give her a box with a prize inside. The box he set down in front of her was made of clear plastic walls fastened together with Velcro. On the top of the box was a bar, and on the front wall was a little transparent door. The box contained a green toy turtle.
Charlotte didn't bother to open the door. She saw a quicker, rougher solution. She ripped the wall from its Velcro and seized the turtle. “I've got it!” she shouted.
“This is an unusual strategy,” Lyons said diplomatically. “The important thing is that she totally ignored the bar.”
A chimpanzee couldn't have done better, I thought.
Next, Lyons showed me Charlotte's trials at the lab. This time an undergraduate named Jennifer Barnes presented Charlotte with another box, with its own combinations of doors and bolts. Unlike the earlier trials, Charlotte now had to watch Jennifer open it first. Barnes had added some extra, useless steps to the process. Jennifer slid the bar back and forth across the top of the box. She picked up a stick and gave the box three careful taps. Only then did she turn the knob on a door, open it, and pull out the toy.
Charlotte's earlier performance had demonstrated that she understood physics well enough to figure out on her own how to open Lyons's boxes. But when Barnes finished her demonstration and handed Charlotte the stick, the box ripper vanished. In her place was a girl driven to imitate every irrelevant step she had been taught.
I could almost hear the chimpanzees hooting. Barnes showed Charlotte four other boxes, and time after time Charlotte followed her useless lesson to the letter.
When the last video ended, I wasn't sure what to say. “SoÂ .Â .Â . how did she do?” I asked.
“She's pretty age-typical,” Lyons said. He had already studied dozens of children, and it was rare that one of them would not perform every step they were taught. Sometimes, Lyons would shake things up by suddenly telling the children that he had to leave soon. Even then, the children wouldn't skip the extra steps. They just performed them faster.
Charlotte's tapping was not a childish mistake. It was actually a little window through which I could glimpse something profound about human nature. We are well
adapted for inheriting culture. We pass genes down through generations, but we also pass down recipes, songs, knowledge, and rituals. Our genetic inheritance endows us with many vital things, from the feet on which we can walk for miles, to the brains we can use to solve problems and think into the future. But if we inherited only our genes, we would not be long for this world. You can't use your brain to reinvent thousands of years of technology and customs on your own.
To say that culture is an important part of our lives doesn't really do the word justice. Culture is not a part of our life. We are a part of it. Lyons's
experiment helped me see how adapted we humans are to immersing ourselves in culture. The Harvard anthropologist Joseph Heinrich has combed through history for unplanned experiments that demonstrate just how dangerous it is for humans to try to survive outside of it. One of the most striking of these experiments took place in 1861, in a region of deserts, mountains, and swamps in eastern Australia, a place called home by a group of Aboriginals known as
The ancestors of the Yandruwandha likely arrived in Australia around sixty-five thousand years ago and then moved into the deep interior over the millennia that followed. Over hundreds of generations, the Yandruwandha built up knowledge about their part of the continent and transmitted it to their children. They came to know where to find the water holes where they could drink and fish. They learned to keep small fires going through the night to survive the outback's winter chill. They gradually figured out how to make bread and porridge from a clover-like fern that they called nardoo that grew in the creeks and swamps of the region.
To dine on nardoo is actually a remarkable feat. Nardoo's cell walls are so tough that you can eat it all day without extracting any nourishment. You will starve even as your stomach feels full and taut. Making matters worse, nardoo contains a toxic enzyme called thiaminase. When it gets into the bloodstream, it destroys the body's supply of thiamine (otherwise known as vitamin B
). People who lose too much thiamine develop a disease called beriberi, which makes them extremely fatigued and shiver with hypothermia as their muscles waste away.
Yet the Yandruwandha could make nardoo a major part of their diet, because they learned how to make it safe to eat. They collected the plant's seedlike sporocarps in the morning and immediately started roasting them in the embers of a fire. The heat destroyed some of the thiaminase, and the ashes likely eliminated more of it by altering the nardoo's pH. The Yandruwandha women then ground the sporocarps between two broad, flat grinding stones, periodically adding water. This procedure inactivated more thiaminase and also broke down the plant's cell walls, turning the nardoo into a digestible flour. The women then used the flour to prepare bread or
porridge. It's customary to eat the porridge with a mussel shell as a spoon. This probably serves as yet another safety measure. If Yandruwandha were to use a leaf instead to eat the porridge, chemical reactions might take place between the leaf and the nardoo that could make it toxic again.
In the summer of 1861, three European men dressed in ragged clothes and leading a sick camel wandered into Yandruwandha territory. Almost a year beforehand, the menâRobert Burke, William Wills, and John Kingâhad marched triumphantly past thousands of cheering people as they left the city of Melbourne. They were part of an eighteen-man crew setting off to do what no European had ever done before: find their way from the southern coast of Australia, through the interior, all the way to the Gulf of Carpentaria on the north side of the continent. The expedition party left with twenty-six camels, twenty-three horses, a two-year supply of food, and a collection of essential equipment for Victorian gentlemen. They even brought enough oak furniture to fill a dining room.
The expedition party made its way across arid wastelands and treacherous swamps. Burke, King, Wills, and a fourth explorer, Charlie Gray, pulled ahead of the rest of the group and stopped about halfway along their journey at a place called Cooper Creek to wait for the rest of their straggling party to catch up. A month passed and no one arrived. Burke, the leader, decided to push forward and leave orders for the rest of the party to wait for them to return.
As the four men continued north, they slowed down even more. At last they reached an estuary filled with salty water. They realized they must be close to the gulf, but they couldn't find the sea. Instead, they kept struggling through mile after mile of coastal wetlands. Exhaustion eventually forced them to turn around and head south without ever setting eyes on the gulf.
The journey south proved even worse. Their supply of food began running out, and the British explorers had little idea how to hunt in Australia for their meals. Gray shot a python, but when he ate it, he contracted dysentery and died. Burke, King, and Wills struggled on toward Cooper Creek for months. When they arrived back at their camp, they found it abandoned. The rest of the explorers had reached Cooper Creek and waited
for three months. When the advance party failed to return, they had headed for home, taking all the food with them. Burke, Wills, and King realized that if they tried to follow the rest of the explorers back south, they would starve. They needed to find help as soon as possible. To the west, they knew there was a cattle station. But to reach it, the three explorers would have to cross an expanse of swamps and then a long stretch of desert. Then they would find the cattle station, near the foot of Mount Hopeless.
Burke, Wills, and King agreed on this dangerous change of plan and headed west. The swamps turned into a maze of creeks, and as they struggled to find the way out, they grew weaker. Their camels suffered even more, dying one by one. Eventually, only one was left. Now their trip across the desert would be doomed, because the sole surviving camel wouldn't be able to carry enough water. It was then, in their greatest desperation, that the explorers met the Yandruwandha.
To Yandruwandha, this place was not a godforsaken wilderness but a place they'd called home for thousands of years. When the Europeans stumbled into the territory, the Yandruwandha assumed they had fallen under a spell that caused them to wander aimlessly, unable to return to the place they had come from. Nevertheless, the Yandruwandha welcomed Burke, King, and Wills. They let the explorers camp by their watering hole and fed them fish and nardoo bread.
Over the next few weeks, the Europeans regained some of their strength. Wills even grew friendly with some of the Yandruwandha. But Burke felt humiliated that they had to accept charity from savages. Surely they shouldn't need help from an inferior raceâtheir superior intelligence alone should have sufficed. This resentment apparently led to conflicts between the travelers and their hosts, causing the Yandruwandha to pack up their camp and slip away.
Burke, King, and Wills were left again to rely only on their intelligence. They tried to fish the local watering holes but couldn't catch anything. It's not clear why they failed where the Yandruwandha succeeded so well. One possibility is that they didn't realize they should use nets like the Yandruwandha. Even if they wanted to use nets, though, they probably had no idea how to fashion them from the local plants.
Without enough fish to eat, the explorers turned to nardoo. They boiled the plants and ate four or five pounds' worth of the stuff every day. But no matter how much they ate, they kept growing more gaunt. “
I am weaker than ever although I have a good appetite and relish the nardu much but it seems to give us no nutriment,” Wills wrote in his journal.
Within a month after the trio parted ways with the Yandruwandha, Wills was dead. Burke and King buried his body and tried to find a way out of the swampy maze. Before long, Burke collapsed and died, and King was now alone. He grew dangerously ill as well as he wandered the swamps, until he ran into a second group of Yandruwandha. They took him in once more. Nestled in their culture, King recovered.
King spent a month in their company before a rescue party from Melbourne found him. They brought King home, where he recounted his misadventures to the city's reporters. News of the expedition quickly traveled around the world, and for years afterward Australians repeated the story until it became a patriotic fable of heroism. Burke and Wills were celebrated with statues, coins, and stamps. Yet their achievement was to have died in a place where others had thrived for thousands of years. The Yandruwandha got no honors for that.
In the years after the Burke and Wills expedition, European anthropologists began scientifically studying the culture of the Australian Aboriginals. They also studied cultures on other continents and remote islands. They would contact isolated groups of people and spend years with them, systematically documenting the words in their languages, their creation stories, their rules for marriage. Anthropologists looked for universal patterns, for variations from place to place. As skeletons of ancient humans came to light in caves, they fit their observations of living cultures into a history of humankind that extended back thousands of years. Victorian anthropologists liked to build a simple history that followed a line of progress predictably ending with their own European culture. But in the early 1900s, as anthropology matured, the line was replaced by a branching tree representing the diversification of cultures over time.