Nicholas E. Curtis and Ray Martinez

The sea slug Elysia chlorotica feeds by sucking the insides out of strands of algae. The slug has taken in the algae’s key tools for using sunlight to help produce food. Researchers now say the slug also can produce — and not just steal — a chemical called chlorophyll, which is needed in that food-production process.

Nicholas E. Curtis and Ray Martinez

For decades, people have been telling each other, “You are what you eat” — meaning that the nutrition in a person’s diet affects his or her health. It doesn’t mean, for example, that if you eat a plant, you become a plant.

At least, not for people.

For a certain kind of sea slug, however, those words are more than just a reminder to eat well. The Elysia chlorotica is a sea slug that looks like a leaf and eats by sucking the insides out of strands of algae. (Yum!) These algae, like plants, get their food by using sunlight to help make sugar.

At a recent meeting of scientists, a biologist named Sidney K. Pierce reported a surprising observation in these algae-eating sea slugs. Pierce does his research at the University of South Florida in Tampa.

Pierce already knew that these sea animals, just like plants, have the right chemical tools to turn sunlight into food. Surprisingly, now he’s learned that the sea slugs aren’t simply stealing what they need to do this from the algae. They’ve also stolen the recipe for how to make chlorophyll, a chemical that is vital to the process, and can make chlorophyll themselves. In other words, they have started to behave like their food.

“This could be a fusion of a plant and an animal — that’s just cool,” John Zardus told Science News. Zardus is an invertebrate zoologist at The Citadel in Charleston, S.C. Invertebrates are animals that don’t have backbones (like slugs), and zoology is the study of animals — so Zardus studies animals without backbones.

Inside their cells, plants have tiny structures called chloroplasts. These chloroplasts turn carbon dioxide and water into sugar using sunlight and a chemical called chlorophyll. (The first part of the word comes from the Greek word chloros, which means “green” —chlorophyll gives green plants their color.) The process of the chloroplasts using chlorophyll to make sugar is called photosynthesis.

Like plants, the algae that get eaten by the sea slugs also use photosynthesis. When Pierce’s slug eats algae, it separates out the chloroplasts. Instead of digesting and excreting the chloroplasts, the sea slug absorbs them inside its own cells. Pierce and his colleagues already knew that once a slug has chloroplasts inside its cells, it can use photosynthesis to make food — which means it may not even have to eat for the rest of its life (about a year). Other animals, like coral, have been known to stash cells containing chloroplasts and use some of the food they make.

But the chloroplasts use up the chlorophyll during photosynthesis, and a fresh supply is needed. Where does it come from? One idea was that when an animal absorbed the chloroplasts, they came with a lifetime supply of chlorophyll. But as it turns out, that’s not the case with these sea slugs. Pierce and his colleagues found that unlike other animals, sea slugs can make their own chlorophyll — which means that they have stolen more than just the chloroplasts.

Deep inside almost every living cell are genes, which function like recipes for how to make what the organism needs. A plant has genes, for example, that contain the instructions for chlorophyll. As it turns out, so do sea slugs — as Pierce and his colleagues are discovering.

So sea slugs not only ingest the chloroplasts — they’ve also “adopted” part of these genetic instructions from their food. In other words, these sea slugs are truly becoming what they eat. Even stranger — it’s the first time the worlds of algae and animals have seemed to overlap like this.

The nerve of one animal

A cancer that spreads from one animal to another has wiped out about 70 percent of the population of Tasmanian devils (one shown).

Image courtesy of Anaspides Photography, Iain D. Williams

A vicious cancer has wiped out 70 percent of the world’s population of wild Tasmanian devils, and if nothing changes, these animals might be extinct in the wild in 30 to 50 years. But there may be hope: In a new study, scientists have identified the cancer and can point to where it starts.

The scientist who led the study is Elizabeth Murchison, who grew up seeing these animals in the wild. “I didn’t want to sit back and let the devils disappear,” she told Science News.

Murchison, who now works at the Wellcome Trust Sanger Institute in Hinxton, England, comes from Tasmania, an Australian island that is the only native home to Tasmanian devils. These animals, which look like small bears, weigh up to 26 pounds, have a long and bushy tail, and often feed on dead animals.

Since 1996, they have been plagued by devil facial tumor disease, or DFTD. As its name suggests, this disease causes large tumors to grow on an infected devil’s face, especially around the mouth. (Devils often bite each other on the face when they meet.) Eventually, the tumors get so large they interfere with eating, and the animal dies of starvation.

To make matters worse, the cancer is contagious. It’s so contagious that some scientists used to believe a virus caused the disease. But that’s not the case — scientists have found that the cancer grows deep in the cells of the central nervous system, and the cancer cells themselves spread from animal to animal through bites (although scientists still don’t know why).

Now that they know where the disease comes from, however, scientists might be able to develop a vaccine that kills the cancer before it becomes deadly, says Katherine Belov. She is a scientist at the University of Sydney, in Australia, who was not involved with the project.

In the study, Murchison and her team reported that the tumors start in cells, called Schwann cells, that usually surround nerve fibers. Nerve fibers, or axons, act like electrical lines in the body: They carry electrical impulses from one neuron (also known as a nerve cell) to the next, or from neurons to muscles.

When the brain sends a signal through the body, the signal travels by way of nerve fibers. Schwann cells wrap themselves around nerve fibers forming a white, wispy layer called myelin. The layer of myelin helps electrical messages zip from one nerve cell to another without shorting out, much like insulation around electrical wires. When a Tasmanian devil’s Schwann cells become cancerous, the animal starts to grow tumors — and becomes contagious.

To understand the cancer, Murchison studied both healthy cells and tumor cells in the animals. In particular, she looked at the genes in the cells. A gene is like a set of instructions for how to build proteins, which are key ingredients that make our bodies work. All the genes together of an organism are called its genome — and a genome is a like a complicated recipe which in humans has more than 20,000 sets of instructions to create its key ingredients.

In a healthy cell, all the genes are working properly. But in a cancer cell, some of the genes aren’t working. Some genes might be “silenced,” or turned off, like a light switch — so they’re not doing their jobs. And other genes might be doing too much of their job — and making too much of one ingredient. (Think about a recipe with too much salt.)

In addition to comparing the genes of healthy and cancerous cells, Murchison looked at microRNAs, which are tiny pieces of genetic material in a cell that help determine which genes get turned on and off. She and her team studied 25 tumors and found they had the same genome (or were “genetically identical”). This led the scientists to conclude that the disease started in a single Tasmanian devil that probably got sick about 20 years ago.

She and her team also noticed that the patterns of genetic activity in the tumor cells (that is, the genetic instructions the cells were following) looked like the patterns found in Schwann cells — which led the scientists to conclude that Schwann cells were the source.

Gregory Hannon, a scientist at Cold Spring Harbor Laboratory on Long Island, N.Y., who worked with Murchison, says that right now, a vaccine that will save the Tasmanian devils is probably a long way off. But that may change in a decade. “Ten years might be enough time” to save the animals, he told Science News.

Mammal babies, including goats, get milk from their mothers.

We put it in cereal. We drink it with cookies. And we eat tons of foods that are made from it, including yogurt, cheese and even some crackers, breads and granola bars. For most of us, milk is a staple that would be hard to live without.

Thousands of years ago, though, only babies drank milk — and that milk came from their mothers. Now, scientists are investigating the beginnings of mankind’s long-lasting love for daily products. They are looking back thousands of years, to the days when people first squeezed milk out of cows and other animals for use as food and drink.

Tracking down the first milk drinkers could give insight into some bigger questions. For example, why do so many people today still get sick from drinking milk? In some countries, almost nobody can digest dairy products.

The work could also help explain major events in human history. Before refrigerators and grocery stores kept a steady supply of fresh food around, dairying probably transformed societies.

“If you can have an animal supply nutrition without killing it, that’s a real step in agriculture,” says Richard Evershed, a chemist at the University of Bristol in the United Kingdom. “That’s spectacular in terms of human nutrition.”

As easy as milk is to find these days, though, its history is challenging to piece together. Like detectives, researchers are tackling the milk mystery in more ways than one.

They are analyzing ancient milk scum on extremely old pots. They’re tracking down the genes that allowed people to digest milk, which is surprisingly hard for many people to stomach. They’re even looking for clues in the buried bones of cows, sheep, horses and other milk-making animals.

“Milk was probably the world’s first superfood,” says Mark Thomas, a scientist at University College London who studies how genes have changed throughout history. The advantages of being able to drink it, he adds, “are just out of this world.”

Thanks, moms

To most people, milk comes in a carton. But milk originally comes from the bodies of mammals. Human as well as other mammal mothers, including dogs, cats, pigs and mice, produce milk to feed their babies.

Mammal babies, including goats, get milk from their mothers. Human mothers also provide milk to their very young children, but most people get milk from the store.

isaact/iStockphoto

Most of the milk in U.S. grocery stores comes from cows. In other countries, it is common to drink the milk of sheep, goats, camels, even horses. Each type of milk has a different flavor. Some types are easier to stomach than others.

Evershed recently sampled milk from horses in Kazakhstan. “It was the most disgusting drink I’ve ever tasted,” he says. “I just didn’t like it.”

Unlike meat, milk does not require that an animal be slaughtered. But the first dairy farmers had to figure out for themselves how to turn wild animals into ones that could be raised in captivity. Then, they needed to herd the animals, care for them and continue to milk them even after the animals’ babies grew up.

Another complication: Milk drinking doesn’t come naturally to older kids and adults. Milk contains a type of sugar called lactose. In order to turn lactose into energy, our bodies need an enzyme called lactase. Enzymes are proteins that help the body do its work.

Like other newborn mammals, baby humans have plenty of lactase, which allows them to gulp down their mothers’ milk. After age 2 or so, though, lactase levels drop.

Without lactase, people can get very sick from dairy products. Symptoms include gas, stomach cramps and severe diarrhea. The condition is called lactose intolerance.

None of our early ancestors could digest milk as adults because their bodies never had to — milk drinking simply wasn’t an option. As people began to extract milk from animals, though, some people developed the ability to keep drinking it throughout their lives.

That biological switch proved to be a huge boost toward survival. Milk is full of calories, fat, protein, calcium and other nutrients. For ancient man, it would have been a valuable and steady source of food.

Scientists now know of a milk-related mutation in our genes — the chemical instructions for life that we carry in almost every cell in our bodies. People who have a mutated form of one particular gene can drink milk just fine. People without the mutation tend to get sick from milk.

“The ability to digest milk, Thomas says, “has been incredibly important for people’s survival for the last 8,000 to 10,000 years. We still just don’t know why.”

The first milk drinkers

To figure out where, and possibly why, milk drinking started, some scientists have been looking at who has the milk-digesting mutation today. Patterns are striking.

Most adults in Northern and Central Europe are able to digest milk — and they do. Cheese, butter and other dairy products are popular in countries such as Sweden, Denmark, Germany and England. Because European settlers dominated North America, most people here can handle milk just fine, as well. That may explain why ice cream is such a popular dessert in the United States.

In much of Africa, Asia and South America, on the other hand, people tend to avoid dairy products because they lead to diarrhea and other stomach problems. (That’s why you won’t typically find cheese on the menu at a Chinese, Japanese or Ethiopian restaurant.) Native Americans are also unable to digest lactose.

Based on these genetic patterns, scientists have long thought that milk drinking started in Northern Europe, where dairy is an institution and the milk-digesting mutation is everywhere.