October 29, 2012
I’m a primate. You’re a primate. Everyone reading this blog is a primate. That’s not news. We hear it all he time: Humans are primates. But what does that really mean? What do we have in common with a baboon? Or a creepy aye-aye? Or even our closest living relative, the chimpanzee?
These are simple questions to answer from a genetic perspective—humans share more DNA with lemurs, monkeys and apes than they do with other mammals. Genetic research of the last few decades suggests that humans and all living primates evolved from a common ancestor that split from the rest of the mammals at least 65 million years ago. But even before DNA analyses, scientists knew humans belong in the primate order. Carl Linnaeus classified humans with monkeys, apes and other primates in his 18th-century taxonomic system. Even the ancient Greeks recognized similarities between people and primates. Today, anthropologists recognize several physical and behavioral traits that tie humans to primates.
First, primates have excellent vision. They have forward-facing eyes that sit close together, which allows the eyes’ fields of view to overlap and create stereoscopic, or 3-D, vision. (In contrast, for example, a cow or giraffe has widely spaced eyes and therefore poor depth perception.) Related to this great eyesight is the presence of a post-orbital bar, a ring of bone that surrounds the eyeball. Many primates also have a completely bony socket that encloses the eye. This bone probably protects the eye from contractions of chewing muscles that run down the side of the face, from the jaw to the top of the head. Many mammals that rely less on vision don’t have a post-orbital bar. If you poked a dog in the side of its head near the temple, you would feel muscle and the eye but no bone (and you would probably be bitten, so please don’t do that). Because primates depend on their vision so much, they generally have a reduced sense of smell relative to other mammals.
Primates are also very dexterous. They can manipulate objects with great skill because they have opposable thumbs and/or big toes, tactile finger pads and nails instead of claws (although some primates have evolved so-called grooming claws on some of their toes). Primates also generally have five fingers/toes on each hand/foot. This is actually a very ancient trait. The earliest mammals had five digits, and over time, many mammalian lineages lost a few fingers and toes while primates kept all of them. Primates also retain collar bones, which allow for greater mobility in the shoulder; mammals that strictly walk on all fours, such as horses, lack collar bones so their limbs are more stable and don’t slip to the side while running.
And in general, primates tend to have larger brains than other mammals of a similar size. They also have smaller litters—often just one baby at a time—and longer periods of gestation and childhood.
Scientists are still trying to understand why primates’ unique set of features evolved. Some researchers think the earliest primates lived in trees, so good vision and dexterity would have been helpful in judging distances between branches or for climbing around. Others, such as Boston University’s Matt Cartmill, have suggested that these traits emerged because early primates might have been insect predators and needed clear eyesight and quick hands to grab prey. Both factors, as well as many others, could have played a role.
October 22, 2012
Everyone’s heard of the A, B, AB and O blood types. When you get a blood transfusion, doctors have to make sure a donor’s blood type is compatible with the recipient’s blood, otherwise the recipient can die. The ABO blood group, as the blood types are collectively known, are ancient. Humans and all other apes share this trait, inheriting these blood types from a common ancestor at least 20 million years ago and maybe even earlier, claims a new study published online today in Proceedings of the National Academy of Sciences. But why humans and apes have these blood types is still a scientific mystery.
The ABO blood group was discovered in the first decade of the 1900s by Austrian physician Karl Landsteiner. Through a series of experiments, Landsteiner classified blood into the four well-known types. The “type” actually refers to the presence of a particular type of antigen sticking up from the surface of a red blood cell. An antigen is anything that elicits a response from an immune cell called an antibody. Antibodies latch onto foreign substances that enter the body, such as bacteria and viruses, and clump them together for removal by other parts of the immune system. The human body naturally makes antibodies that will attack certain types of red-blood-cell antigens. For example, people with type A blood have A antigens on their red blood cells and make antibodies that attack B antigens; people with type B blood have B antigens on their red blood cells and make antibodies that attack A antigens. So, type A people can’t donate their blood to type B people and vice versa. People who are type AB have both A and B antigens on their red blood cells and therefore don’t make any A or B antibodies while people who are type O have no A or B antigens and make both A and B antibodies. (This is hard to keep track of, so I hope the chart below helps!)
After Landsteiner determined the pattern of the ABO blood group, he realized blood types are inherited, and blood typing became one of the first ways to test paternity. Later, researchers learned ABO blood types are governed by a single gene that comes in three varieties: A, B and O. (People who are type AB inherit an A gene from one parent and a B gene from the other.)
More than a hundred years after Landsteiner’s Nobel Prize-winning work, scientists still have no idea what function these blood antigens serve. Clearly, people who are type O—the most common blood type—do just fine without them. What scientists have found in the last century, however, are some interesting associations between blood types and disease. In some infectious diseases, bacteria may closely resemble certain blood antigens, making it difficult for antibodies to detect the difference between foreign invaders and the body’s own blood. People who are type A, for instance, seem more susceptible to smallpox, while people who are type B appear more affected by some E. coli infections.
Over the last hundred years, scientists have also discovered that the ABO blood group is just one of more than 20 human blood groups. The Rh factor is another well known blood group, referring to the “positive” or “negative” in blood types, such as A-positive or B-negative. (The Rh refers to Rhesus macaques, which were used in early studies of the blood group.) People who are Rh-positive have Rh antigens on their red blood cells; people who are Rh-negative don’t and produce antibodies that will attack Rh antigens. The Rh blood group plays a role in the sometimes fatal blood disease erythroblastosis fetalis that can develop in newborns if an Rh-negative women gives birth to an Rh-positive baby and her antibodies attack her child.
Most people have never heard of the numerous other blood groups—such as the MN, Diego, Kidd and Kell—probably because they trigger smaller or less frequent immune reactions. And in some cases, like the MN blood group, humans don’t produce antibodies against the antigens. One “minor” blood type that does have medical significance is the Duffy blood group. Plasmodium vivax, one of the parasites that causes malaria, latches onto the Duffy antigen when it invades the body’s red blood cells. People who lack the Duffy antigens, therefore, tend to be immune to this form of malaria.
Although researchers have found these interesting associations between blood groups and disease, they still really don’t understand how and why such blood antigens evolved in the first place. These blood molecules stand as a reminder that we still have a lot to learn about human biology.
October 15, 2012
I was sick this weekend. The kind of sick where your nose runs so much that you begin to question how the human body can produce so much mucus. My throat hurt. I was coughing. But the worst part was the headache: My head felt like it was being continuously squeezed by a vise, or maybe some sort of medieval torture device. The pain was so bad even my teeth hurt. As I was lying in bed next to my half-empty box of Kleenex, I thought, “This wouldn’t be happening if we had descended from Asian, not African, apes.” (Yes, I was really thinking that.)
But before I explain what apes have to do with my cold, let’s cover some basic biology. When the cold virus (or bacteria or an allergen like ragweed) enters the body, the nose produces mucus to prevent an infection from spreading to the lungs. This results in a runny nose. All of the extra snot can also plug up passages that connect the nose to air-filled pockets in the bones of the skull, called sinuses. Sinuses produce their own mucus and are thought to help humidify air, as well as stabilize and strengthen the skull. But when the passageways between the head’s sinuses and nasal cavity get blocked, the sinuses’ mucus can’t drain and the air pockets fill, causing pressure to build . Sometimes the lining of the sinuses swell, which results in the further production of mucus and build-up of pressure. That pressure hurts.
Humans have four types of sinuses that play a role in sinus headaches: the frontal sinus in the forehead, the maxillary sinus in the cheeks, the ethmoid sinus between the eyes and the sphenoid sinus behind the nose. The African apes, gorillas and chimpanzees, have all four of these sinuses. The Asian apes, orangutans and gibbons (the so-called lesser apes because of their smaller size), have just two, lacking the ethmoid and frontal sinuses.
The ethmoid and frontal sinuses can be traced back at least 33 million years ago to a primate called Aegyptopithecus that lived in Africa before the ape and Old World monkey lineages originated. (Old World monkeys are those that live in Africa and Asia.) These sinuses have also been found in some of the earliest known apes, such as the roughly 20-million-year-old Morotopithecus and 18-million-year-old Afropithecus, both from Africa. Chimpanzees, gorillas and humans inherited these sinuses from the most ancient apes. Gibbons and orangutans, however, each lost these sinuses independently after they diverged from the rest of the apes; gibbons evolved about 18 million years ago while orangutans split from the other great apes roughly 15 million years ago.
It’s not clear why the Asian apes lost the ethmoid and frontal sinuses. In the case of the orangutan, the animal has a much more narrow space between its eyes and a more severely sloped, concave forehead than the African great apes. So there just may not be room for these air pockets to form.
But gibbons and orangutans do still have the maxillary and sphenoid sinuses, which are enough to cause annoying pain and headaches. So I should really apologize to my African ape ancestors. Clearly, I had some misdirected anger. I should have been mad at the virus that invaded my body.
October 10, 2012
Ethiopia may well deserve the title Cradle of Humankind. Some of the most famous, most iconic hominid fossils have been discovered within the country’s borders. Ethiopia can claim many “firsts” in the hominid record book, including first stone tools and the first Homo sapiens. Here’s a look at the country’s most important hominid finds.
Omo I and II (1967-1974): While excavating the Kibish Formation near the Omo River, Richard Leakey and his colleagues uncovered a partial skull and skeleton (Omo I) and a partial skull (Omo II) that are still thought to be the oldest examples of Homo sapiens. Dating to 195,000 years ago, Omo I has several features that clearly place it within our species, including a flat face, high forehead and prominent chin. Omo II, on the other hand, looks more primitive. While some researchers suggest its thicker skull and sloped forehead preclude it from being a true modern human, others say those features were probably within the range of variation for early H. sapiens.
Lucy (1974): While searching a dry gully at the site of Hadar, paleoanthropologist Don Johanson noticed a slender arm bone sticking up from the ground. He thought it belonged to a hominid. Then he noticed a thigh bone, some bits of a spine, a pelvis and some ribs. Eventually, Johanson and his colleagues unearthed approximately 40 percent of a hominid skeleton dating to roughly 3.2 million years ago. Named Lucy after the Beatles’ “Lucy in the Sky with Diamonds,” the skeleton is officially known as AL 288-1 and is arguably the most famous hominid fossil ever found. But it took a while for Johanson, with the help of paleoanthropologist Tim White, to figure out what Lucy was—Australopithecus afarensis—and her place in the human family tree. (For a firsthand account of Lucy’s discovery and the analysis of her remains, you probably can’t find a better book than Lucy: The Beginnings of Humankind by Johanson and Maitland Edey, even if some of the science is out of date.)
First Family (1975): Just a year after discovering Lucy, Johanson’s team got lucky again, finding a jumble of more than 200 A. afarensis fossils at the site of Hadar. The collection—representing as many as 17 individuals—was dubbed the “First Family” (official name: AL 333). Because the fossils contained both adults and youngsters, the First Family is a snapshot of variation within A. afarensis and offers a look at how an individual within the species might have grown up. Anthropologists are still trying to figure out what led to the demise of such a large group of hominids. A catastrophic flood is one theory; death by over-eager carnivores is another.
Australopithecus garhi (1990, 1996-1998): Paleoanthropologists Berhane Asfaw and Tim White found a partial skull and other pieces of the 2.5-million-year-old species known as A. garhi in 1990 at the site of Bouri. Since then, no additional fossils have been unearthed (or, at least, matched to the species). Not much is known about A. garhi. Based on the length of a thigh bone, the species may have had slightly longer legs, and therefore a longer stride, than Lucy’s kind. Given the species’ age and where it was found, A. garhi may have been the hominid to make the oldest known stone tools (described next).
Oldest Stone Tools (1992-1994): At 2.6 million years old, the stone choppers, or Oldowan tools, at the site of Gona are a few hundred thousand years older than any other known stone tool. But the Gona tools’ status as earliest stone tool technology was recently challenged by another Ethiopian discovery. In 2010, archaeologists claimed that roughly 3.39-million-year-old mammal bones from Hadar contained scratches that could have only been made by a stone tool, implying stone tools were an even earlier invention than scientists had thought. Other researchers remain unconvinced that the markings were made by hominid butchering. And since no actual stone tools were found along with the bones, the Gona artifacts’ title of earliest known stone tools is still safe.
Ardi (1992-1994): Older than Lucy, Ardi is the most complete skeleton of an early hominid. The first pieces of the 4.4-million-year-old Ardi were uncovered in 1992 by one of Tim White’s graduate students, Gen Suwa, in the Middle Awash Valley. White and his colleagues then spent more than 15 years digging Ardi out and analyzing the skeleton. The hominid did not look like Australopithecus, so the researchers gave it a new name: Ardipithecus ramidus. Although the species walked upright on two legs, its form of bipedalism was quite different from that of modern people or even Lucy. Its discoverers think Ardipithecus represents an early form of upright walking and reveals how apes went from living in the trees to walking on the ground.
Ardipithecus kadabba (1997): Yohannes Haile-Selassie of the Cleveland Museum of Natural History unearthed hand, foot and other bones in the Middle Awash Valley that looked a lot like those of Ar. ramidus—only the bones were almost a million years older, with an age of about 5.8 million years. Teeth found in 2002 suggested the more ancient hominids deserved their own species: Ar. kadabba. It remains one of the earliest known hominid species.
Dikika Child (2003): From the site of Dikika comes the fossil of an approximately 3-year-old A. afarensis child dating to 3.3 million years ago. Sometimes called Lucy’s baby or Selam, it’s the most complete skeleton of an early hominid child, including most of the skull, torso, arms and legs. The fossil’s discoverer, Zeresenay Alemseged, of the California Academy of Sciences, and colleagues say the fossils suggest A. afarensis grew up quickly like a chimpanzee but was beginning to evolve slower growth patterns like those of modern humans.
Herto fossils (2003): Even if the Omo I and II fossils turned out not to be members of H. sapiens, Ethiopia would still be home to the earliest known members of our species. A team led by Tim White discovered three 160,000-year-old skulls in the Middle Awash Valley. Two belonged to adult H. sapiens while the other was of a child. Due to some features not seen in modern populations of humans, White and his colleagues gave the skulls their own subspecies: H. sapiens idaltu.
Australopithecus anamensis (2006): A. anamensis, the earliest species of Australopithecus, was already known from Kenya when a team led by Tim White of the University of California, Berkeley discovered more fossils of the species further north in Ethiopia’s Middle Awash Valley. The collection of roughly 4.2-million-year-old fossils is notable because it includes the largest hominid canine tooth ever found and the earliest Australopithecus femur.
September 26, 2012
By 200,000 years ago, Homo sapiens had emerged somewhere in Africa. By 14,000 years ago, our species had spread to every continent except Antarctica. What happened in between—the pattern of where humans went and when—is still being worked out. To reconstruct the peopling of the world, anthropologists rely on several types of clues.
Fossils: The most obvious way to track our ancestors’ movements is to look for their physical remains. Researchers sketch out travel routes by mapping where the oldest human fossils are found. The earliest Homo sapiens bones outside of Africa come from a cave site in Israel called Qafzeh. Here the skeletons of both adults and children date to as far as 125,000 years ago. This first foray out of Africa didn’t last long. Humans disappeared from the fossil record outside of Africa for many tens of thousands of years, perhaps because the climate became too harsh. Fossils tell us humans made a successful, sustained exodus by at least 50,000 years ago. Human fossils found at Australia’s Lake Mungo site, for example, have been dated to between 46,000 and 50,000 years ago (PDF).
The problem with relying on skeletal remains to map early migrations is that the timing of our ancestors’ travels is only as good as the methods used to date the fossils. Sometimes scientists find bones in places that are not easily dated by geological techniques. And in some areas, fossils aren’t prone to preservation, so there are probably huge gaps in our knowledge of the paths early humans took as they spread around the world.
Artifacts: Archaeologists also look for the items people made and left behind. For example, stone tool discoveries suggest an alternative route out of Africa. For decades, scientists assumed humans left Africa via the Sinai Peninsula, but in the last several years some researchers have favored a “southern” route: leaving from the Horn of Africa, crossing the narrowest part of the Red Sea and entering into southern Arabia. Last year, archaeologists reported finding stone tools in Oman dating to roughly 106,000 years ago. At that time, the Arabian Peninsula was a much more hospitable place than it is today, home to numerous freshwater lakes. As the region became drier, people might have moved east into Asia or returned to Africa.
Of course, when the only remains at an archaeological site are tools, it’s hard to say with absolute certainty who made them. The researchers working in Oman noted that the tools they found in Arabia match the technology of modern humans found in eastern Africa about 128,000 years ago. The team made the case that the tool makers on either side of the Red Sea belonged to the same cultural group—and therefore the same species. But as anthropologists discover more species, such as the Hobbit or the Denisovans, that lived alongside modern humans outside of Africa up until a few tens of thousands of years ago, it becomes harder to say stone tools alone indicate the presence of Homo sapiens.
DNA: Genetic data can help fill in the holes in the human migration story that fossils and artifacts can’t address. Anthropologists collect DNA samples from different ethnic groups around the world. Next, they count up the genetic differences caused by mutations in certain sections of the genome. Groups that are more closely related will have fewer genetic differences, which implies they split off more recently form each other than they did with more distantly related groups. Scientists calculate when in the past different groups diverged from each other by adding up all of the genetic differences between two groups and then estimating how often genetic mutations occurred. Such analyses not only give a sense of when different parts of the world were first inhabited, but they can also reveal more intricate patterns of movement. For example, genetic data suggest North America was colonized by three separate waves of people leaving Siberia across the Bering Strait.
Genetic data are not foolproof, however. The estimated divergence times are only as accurate as the estimated mutation rate, which scientists still debate. In the early days of DNA studies, scientists used either mitochondrial DNA, passed down only by the mother, or the Y chromosome, inherited only from father to son. Neither of these types of DNA presented the full picture of what people were doing in the past, as mitochondrial DNA only tracks maternal lineages while the Y chromosome only follows paternal lines. Today, whole genome sequencing is beginning to allow researchers to trace entire populations.
Languages: Anthropologists use languages in methods analogous to studying DNA; they look for patterns of similarities, or differences, in vocabularies or other aspects of language. Earlier this year, researchers compared different languages within the Indo-European language family to determine where these languages arose. After assessing the relationship between the languages, the researchers considered the geographic ranges where those languages are currently spoken. They concluded that the Indo-European language family originated in what is today Turkey and then spread west into Europe and east into southern Asia as people moved into these areas. But such linguistic analyses may only track relatively recent migration patterns. For example, H. Craig Melchert, a linguist at the University of California, Los Angeles, told Science News that the Indo-European languages can only be traced back about 7,000 years.