At the most fundamental level, we're nearly identical to apes. Humans share 99 percent of our genetic information with chimpanzees, our closest relatives, and 93 percent with rhesus macaques, one of the most common monkey species. Out of our roughly 20,000 genes, less than 50 are exclusive to humans. From such tremendous similarity, how do our substantial differences arise? 

The PsychENCODE Consortium—a U.S. government-funded initiative, involving 116 researchers at 15 institutes—is trying to finally come up with an answer. The collective recently published their findings as a series of 11 papers in a special issue of Science. One of these studies, spearheaded by Yale neuroscientist Nenad Sestan, radically redefined our understanding of how our brains become human.

Sestan’s team studied both humans and rhesus monkeys as they grew up, from infancy into early adulthood. The researchers followed how the human and monkey brains changed over time, using a technique known as single cell RNA sequencing. This allowed them to isolate individual cells from the brains at different ages, profiling the amount of every gene in each cell. This means that the scientists could track changes, uncovering differences that are masked when millions of cells are combined, as they are in traditional genetic tests. 

The amount of a gene in each cell can matter just as much, if not more, as the identities of the genes themselves. Consider baking cookies: Two recipes can have all the same ingredients, but if one calls for twice as much salt or baking powder, the final results will be dramatically different. If what separates people from primates is a result of how many genes of a certain type are in certain groups of cells—well, this would be the first study that could possibly detect these differences.

The researchers quickly noticed that for the most part, humans and monkeys don't actually diverge much as they're growing up. Humans go through every stage of macaque development, and most aspects of brain growth happen at the same time across the two species. In both humans and monkeys, brain cells—including neurons, cells that send messages to and from the brain, and glia, the cells that support neurons by providing nutrients and protection—develop and diversify in the same order, and at about the same time. As a result, our brains don't just grow in parallel—during certain periods, they actually become more similar to each other as they develop. 

The next logical question, is what, exactly, causes humans to deviate so radically from monkeys? The Yale researchers found a few notable differences: Though humans advance through all stages of macaque development, humans also have two extra stages of brain development, ones found in no other animal. The first comes at the very beginning of life, when fetuses experience a short period of great change. Interestingly, during this phase fetuses show developments in their prefrontal cortex, which is  crucial to complex thought and decision making, the most distinctive parts of human thought, and an area of the brain that's comparatively. After this period, our brains  rapidly become more similar to those of monkeys—until the next period of divergence.

Sestan’s team found this second period comes during late childhood, around the age of ten. Humans have a prolonged childhood, meaning we take longer to mature, and also keep some childlike traits, like having large eyes and heads, permanently. But until now, scientists didn't understand why we take so long to develop.  Only around the end of elementary school do the brain cells from our two species begin to form connections and transmit information differently. As a result, the researchers speculate that this phase could be responsible for how humans learn, think, and remember differently than monkeys.

Surprisingly, in all stages of development, humans and monkeys shared most cell types. This means if a neuron is present in humans, it is likely present in monkeys too. Our two species' genetic differences are generally found in a few small groups of cells, mostly subgroups of neurons. But even in these, the genetic differences across species remained relatively small—in total, the researchers found out of 20,000 genes, only about 50 diverged significantly. Despite the small number, some of these genes play major roles in mental function; many have been linked to disorders such as schizophrenia or bipolar disorder. For example, the gene GRIA1 controls learning and memory, causing schizophrenia by distorting the underlying basis of thought.

Even though this work represents a major step forward in our understanding of what makes humans unique, this study only examined the genes found in a cell. Human and monkey brains may also differ in many other ways—like how brain cells connect to one another, or how these cells  communicate. Further research along these lines will be necessary to fully understand the relationship between our two species. 

Regardless, Sestan’s study illustrates how seemingly minor changes can yield major effects. Only two short periods of development meaningfully separate the brains of humans and monkeys, where only a few cell types receive the brunt of these changes. A minute number of genes diverge in just a few cells, and suddenly, we acquire the ability to read, to write, to speak—to be human. 

After the Planet of the Apes came out, there was some speculation on what hypothetically could've made these apes become so close to humans, and what such a scenario would require in real life. We now finally have an answer: not much at all.

Everyone has their own reasons, but surveys have revealed that three broad concerns appeared to be nearly ubiquitous:

1. The vast majority of vegetarians changed their diet for health reasons.
2. Many vegetarians also adopted their diet for ethical reasons — two-thirds described feelings of obligation to protect animals or disgust towards meat products as a motivator to switch.
3. Finally, 59 percent of vegetarians preferred their diet for its environmental impact.

Concern over antibiotic resistance due to use in livestock has also grown as new revelations concerning the dangers of conventional (livestock-derived) meat appear in headlines daily. Livestock and meat serve as a major source of disease outbreaks today. Bird flu, anthrax, and swine flu all incubate in and spread from domesticated poultry, cows, and hogs to humans. Over 60 percent of human pathogens are zoonotic, meaning both humans and animals can contract them. In a recent example from March 2018, a listeriosis outbreak from processed meat killed upwards of 200 people and infected 1,000 more. Conventional meat’s negative health impacts do not end at infectious disease, though. Long-term processed meat consumption is associated with increased heart disease, digestive tract cancer, and type 2 diabetes across all demographic groups.

But soon, people may not need to make such drastic dietary changes if they have these concerns. Instead, by eating meat grown in a lab, people could continue to enjoy meat products while gaining some of the major advantages of a vegetarian diet. Strict environmental controls and tissue monitoring can prevent infection of cultures from the outset, and if any do get infected, they could potentially be caught before shipping to consumers. Lab-grown meat can also leverage advancements in biotechnology, eventually including increased nutrient fortification, individually-customized cellular and molecular compositions, and optimal nutritional profiles, giving it the potential to be far healthier than livestock-derived meat.

Additionally, lab-grown meat may successfully resolve long-running ethical dilemmas intrinsic to a carnivorous diet. Many ethicists believe that eating meat results in a great deal of unnecessary pain and suffering for animals. When presented with moral arguments against meat consumption, carnivores outside of academia do tend to agree with the ethicists—though not enough to change their behavior. For example, celebrity chef and noted meat connoisseur Anthony Bourdain struggled mightily with what he perceived as a major contradiction between his morality and his career. Lab-grown meat could allow these people to continue enjoying meat without lingering moral concerns.

Finally, many people simply lack the choice to forgo meat, no matter what their conscience may dictate.

In short, the process's ethical advantages result from growing in isolation in a dish in an incubator. Muscle cells cannot think or experience anything on their own, and they do not independently develop a brain. Without consciousness, cultured meat cannot suffer. It renders the (previously unavoidable) issue of animal suffering moot, to the same degree that a vegetarian diet would.

Regardless of moral philosophy, what's less controversial is that conventional meat production exerts a tremendous cost on the environment. It requires 30 percent of the Earth’s surface, as well as one third of its freshwater, while producing 18 percent of global greenhouse gas emissions. It takes 15 to 100 kilograms of plant matter and 41 kilograms of carbon dioxide to produce one kilogram of meat. For many nations signed to the Paris Agreement, "meat and fish companies may be 'putting the implementation of the Paris agreement in jeopardy' by failing to properly report their climate emissions."

Though they disagree on the overall degree, the two existing environmental impact studies (one, two) concur lab-grown meat has the potential to be much more efficient than conventional meat production. It's important to note these estimates were made in the infancy of meat culturing, before methods have matured, economies of scale have been established, or regulation has begun. As development continues, we can only expect meat cultures to become more efficient and the comparison even more favorable. In fact, the cost of production per kilogram has already dropped by a factor of over 4,000 in just a few years, indicating large initial increases in efficiency and scale. But current cutting-edge techniques still have a lot of room for improvement, too.

Given that roughly 95 percent of the American population eats meat (and approximately 97 percent of the UK), cultured meat stands to have a large customer base. Former vegetarians and vegans usually list boredom, social and logistical difficulties, and meat cravings as the primary reason for adding meat back into their diet; more than 80 percent of vegetarians and vegans go back to eating animal products, the majority after less than a year.

Indeed, taste is the major deciding factor for consumers who buy plant-based meat substitutes. Looking at the history of plant-based substitutes, we see the same concerns driving nearly all development. Chefs designing new plant-based meat substitutes have focused almost exclusively on making them more closely resemble meat in taste, texture, and appearance.

Tofu is the oldest and most popular meat substitute both today and throughout history. It arose in China over a thousand years ago, following the spread of Buddhism into the country. Some Buddhists believe meat shouldn't be consumed, largely due to animal cruelty. Legend has it that Chinese chefs invented tofu to replace meat in most dishes, calling it “vice mayor’s mutton.” It quickly became popular in China and spread throughout East Asia, where it remains a staple to this day.

Chefs in Asia developed a second substitute in the fifth century CE. By washing and kneading wheat dough until all the starch came out, they produced loaves of gluten, the main protein in wheat. The loaves have a stringy texture and a savory taste, similar to poultry, and became known as seitan. The loaves can be cut and seasoned to closely resemble slices of meat in both taste and texture. Seitan has become a staple in Asian cuisine, referred to as various “mock” meats, most commonly mock duck and mock chicken. Though seitan resembles meat far more closely than tofu, it still doesn't stand in well for red and processed meats.

Then in the U.K. in the 1990s, food manufacturer Quorn entered the mock meat marketplace. They produce frozen foods made from mycoprotein, a form of protein sourced from single-celled fungi that is easily bound together and pressed into various shapes. Using mycoprotein, they produce substitutes for most processed and frozen meats, including nuggets, burgers, and meatballs, with similar appearances, textures, and flavors. Mycoprotein can only faithfully reproduce the features of these lower-grade frozen meats, limiting the range of their approach.

We’re in the midst of the most recent wave of meat substitute development, which began in the early 2010s. Two companies are attempting to closely replicate the feeling of unfrozen, unprocessed cuts of meat, using burgers as an initial proof-of-concept. The first company, Beyond Meat, uses pea protein mixed with fats in the same proportion as found in ground beef, allowing them to mimic the taste and feel of conventional ground beef and burgers. The second company, Impossible Foods, uses mixes of different protein and fat molecules to find tastes and textures most similar to meats products. They then add plant-derived heme, a molecule in blood that gives it its red color and distinct smell, to give a more natural, bloody aesthetic to their meats. Both of these companies have raised tens of millions of dollars from investors and begun to roll their products out to various grocery stores and fast food chains.

Both of these substitute meat products closely match the appearance, texture, and taste of regular meat, addressing the most common concern about going meatless. Consumers appear to agree: multiple surveys have shown that 40 to 70 percent of people would be willing to switch to cultured meat.

So when we discuss alternatives to conventional meat, we are not discussing a brand-new phenomenon. Cultured meat is the next step in a long development process, and could replace conventional meat with a healthier, more ethical, and more sustainable alternative.

(Ed. note: This article erroneously referred to Beyond Meat and Impossible Foods products as being "cultured" meat products. They are not cultured meat, instead being made of vegetable products made to imitate meat. This has been corrected. We regret the error.) 

There are two crucial details to this story: Hof began his hike at Base Camp on Mount Everest, and the pain in his foot was caused by severe frostbite. He had reason to think he could withstand the extreme conditions; Wim Hof is also known as "The Iceman," holder of 26 world records and one of the most successful extreme athletes of all time. He attributes his success to a breathing method that he thinks exploits his "reptilian brain," helping him acquire a superhuman tolerance to punishing cold. According to some, tricks like these fool the lizard part of your brain – the more primitive, unconscious mind – and can be used to make us vulnerable to marketing, lose us money, or maybe even elect Donald Trump.

Paul MacLean first proposed the idea of the “lizard brain” in 1957 as part of his triune brain concept, theorizing that the human brain supposedly consists of three sections, nested based on their evolutionary age. He believed the neocortex, which he thought arose in primates, is the largest, outermost, and newest part of the human brain: It houses our conscious mind and handles learning, language, and abstract thought. MacLean thought the older, deeper limbic system – which mediates emotion and motivation – began in mammals. Finally, he traced the brainstem and basal ganglia back to primordial reptiles, theorizing that they controlled our reflexes, as well as our four major instincts: to fight, flee, feed, and fornicate.

His ideas spread like wildfire from the ivory tower, setting the world below ablaze. The triune brain theory soon became central to most people's understanding of our primordial ancestors' minds, including influential thinkers like Carl Sagan and Arthur Koestler. Even today, many people still think about the brain on MacLean's terms. But as enthusiasm grew in popular audiences, so did discontent among other scholars. Among other issues, critics claimed MacLean's groupings were too simplistic, and failed to account for birds, who display remarkable intelligence, despite possessing brains that are somewhat similar to lizards'. However, all these issues amounted to academic disagreements, with little real evidence undermining MacLean's theories.

That is, until May, when these dissenters witnessed a discovery they had long anticipated. Gilles Laurent, a world-renowned reptile neuroscientist at the Max Planck Institute, published a landmark study in Science with a relatively humble conclusion: The differences between the brains of reptiles and humans seem largely overstated. These findings suggest that MacLean's theories about the divisions of our brains were off-base. They also raise a provocative question: if MacLean's theories have held sway for so long, despite numerous detractors, why was such a similarity only discovered now?

The answer, like most others in neuroscience, is largely technological. Laurent's experiments required a very recent, cutting-edge technology to uncover these similarities: single cell RNA-sequencing (scRNA-seq), which has only became widely available in the past three years. The technique measures the overall production of most genes in isolated single cells, allowing simultaneous profiling of individual cells based on a combination of nearly all of their genetic characteristics. These abilities give scRNA-seq uniquely high power and unparalleled resolution, allowing scientists to group cells together based on their combined genetic similarities, as well as separate them based on their genetic differences.

Laurent's group sequenced hundreds of genes from more than 20,000 cells from two of the most common domestic reptiles: turtles and bearded dragons. These cells came from the pallium, the brain region in lizards that Laurent thought most likely to resemble the neocortex, which MacLean thought didn't exist in lizards. They could then compare the genetic signatures of these cells to those from human and mouse brains, the two best-studied species in single-cell genetics studies. Luckily for them, these two species neatly correspond to MacLean's two other proposed divisions of the brain! Cells in the mouse brain could represent the proposed mammalian division, and cells in the human brain could represent the primate brain, allowing the researchers to put MacLean's theories to the test.

Human, mouse, lizard

Their first comparison was relatively simple. We know so little about the brains of these lizards that the researchers had to find out what kinds of brain cells lizards have in the first place, and whether they are close to our own genetically. By looking at genes in human and mouse neurons — the brain cells that store information — and the glia, the cells that support the neurons, the scientists found that reptiles' brain cells weren't actually all that different from our own.

Knowing that the three species' brain cells were molecularly similar, with the same broad categories of cells acting the same way, Laurent's group then compared the region-specific properties of neurons in the different parts of their brains. They compared neurons in the reptiles' pallium – the upper layers of cells in the brain – with those of mouse and human neurons from the neocortex. Then, they compared the reptile neurons with two parts of the mammals' limbic system, the hippocampus and amygdala, which usually process learning and fear, respectively. 

There, the scientists observed something unexpected: Two large pallial regions, the medial and dorsoventral pallium, were respectively very similar to the mammals' hippocampus and amygdala. When they looked closer at specific subregions of the hippocampus, they could even identify specific subregions of the medial pallium that were very similar to specific subregions of the hippocampus.

Surprisingly, even when they compared subregions of the pallium to the neocortex, they uncovered the same pattern in these neurons as well. Cells in just one small part of the pallium, the anterior dorsal cortex, displayed a remarkable similarity to cells in the human neocortex. When they looked at the types of cells in these similar regions, the cell types present in one were generally present in the other too. However, the similarities were far from universal. While some neocortex-specific classes of neurons seemed almost exactly the same in lizards, mice, and humans, some other classes of neocortical neurons in humans weren't present in lizards. For example, many types of human neurons have properties specific to their individual layers, while lizard neurons do not, implying a greater degree of specialization.

Taken together, these comparisons showed that rather than considering the reptilian brain a distinct unit, separate from the mammalian limbic system and the primate neocortex, the three can't be cleanly distinguished from one another. Instead, reptiles have primitive versions of both of MacLean's "higher" brain areas, all but proving his theories false. These areas didn't evolve from scratch after reptiles, but instead simply expanded out from their smaller, less well-defined reptilian precursors.

These findings don't just change how we classify regions of the brain, they change how we think about lizards and other "lower" animals. If their brains are somewhat similar to our own, why wouldn't their thought be similar as well? If lizards do think so differently, what relatively minor genetic difference could possibly cause such a dramatic change? 

These questions have no real answers yet, and are sure to spark vigorous debate and research in neuroscience, psychology, and philosophy in the months and years ahead.