kottke.org posts about humans
How old are different parts of our bodies? Does anything stick around the entire time? The hair on our bodies lasts only a few years. Fingernails are fully replaced every six months. Your skin lasts 2-4 weeks. Even your blood and bones regenerate every so often. There's at least one part of your body with lasts the whole time you're alive, which I found somewhat surprising. See the ship of Theseus paradox.
The ship wherein Theseus and the youth of Athens returned from Crete had thirty oars, and was preserved by the Athenians down even to the time of Demetrius Phalereus, for they took away the old planks as they decayed, putting in new and stronger timber in their places, in so much that this ship became a standing example among the philosophers, for the logical question of things that grow; one side holding that the ship remained the same, and the other contending that it was not the same.
How do we know the lifespans of different cells in the body? Carbon-14 levels from nuclear testing done in the 50s and 60s.
Analysis of growth rings from pine trees in Sweden shows that the proliferation of atomic tests in the 1950s and 1960s led to an explosion in levels of atmospheric carbon 14. Now, Jonas Frisen and colleagues at the Karolinska Institute in Stockholm have taken advantage of this spike in C14 to devise a method to date the birth of human cells. Because this test can be used retrospectively, unlike many of the current methods used to detect cell proliferation, and because it does not require the ingestion of a radioactive or chemical tracer, the method can be readily applied to both in vivo and postmortem samples of human tissues.
Kurzgesagt gives us a short tour of human history, from the six different species of human that existed 100,000 years ago to the present. If you found that interesting and want more detail, you should read Sapiens...Kurzgesagt used it as a major reference here.
Most books about the history of humanity pursue either a historical or a biological approach, but Dr. Yuval Noah Harari breaks the mold with this highly original book that begins about 70,000 years ago with the appearance of modern cognition. From examining the role evolving humans have played in the global ecosystem to charting the rise of empires, Sapiens integrates history and science to reconsider accepted narratives, connect past developments with contemporary concerns, and examine specific events within the context of larger ideas.
Kurzgesagt and CGPGrey collaborated on a pair of videos about the self. The first video considers the human being as a collection of cells. How many of those cells can you take away before you stop being you? And does that question even make sense? The second video notes that if you sever the connection between the two halves of the human brain, they will each seemingly continue to operate as separate entities. But which of those entities is you? Are there two yous?
In the 90s, Bruniquel Cave was discovered to have a chamber containing an interesting human-built structure made from broken stalagmites. Carbon dating of a burnt bear bone within the chamber put the age of the activity at 47,600 years ago, smack dab in the Neanderthal era in that area. But recently, after a lull in research about these cave structures, analysis of uranium levels in the broken stalagmites resulted in a much older date for the construction: 176,500 years ago.
Nor is it clear how the Neanderthals made the structures. Verheyden says it couldn't have been one lone artisan, toiling away in the dark. Most likely, there was a team, and a technically skilled one at that. They broke rocks deliberately, and arranged them precisely. They used fire, too. More than 120 fragments have red and black streaks that aren't found elsewhere in the chamber or the cave beyond. They were the result of deliberately applied heat, at intensities strong enough to occasionally crack the rock. "The Neanderthal group responsible for these constructions had a level of social organization that was more complex than previously thought," the team writes.
There are some things that humans don't need to survive anymore still hanging around on our bodies, including unnecessary arm muscles and vestigial tail bones.
From the landmark science series Cosmos, Carl Sagan narrates the evolution of humans from the first cells billions of years ago.
There's a new (extinct) hominin on the block (in a cave system in Africa): Homo naledi.
Basically, Homo naledi had humanlike hands, legs, and feet, but tiny little australopithecine skulls, i.e., brains.
And, there are a lot of them, fossil-wise: "Representing at least 15 individuals with most skeletal elements repeated multiple times, this is the largest assemblage of a single species of hominins yet discovered in Africa."
But we don't yet know exactly how long ago they lived: maybe two million years ago, when the Homo genus emerged, or as late as a hundred thousand years ago, when it contracted to, well, us. In an accompanying article, Chris Stringer gets a little testy about it.
Frustratingly, the rich and informative H. naledi material remains undated. Given that this hominin material could conceivably even date within the last 100,000 years, I am puzzled by the apparent lack of attempts to estimate its age. This could have been achieved directly via radiocarbon dating (even if only to test whether the material lies beyond the effective range of that method) or indirectly based on ancient DNA samples. For example, after ancient DNA was successfully recovered from the Sima de los Huesos fossils, it was used to date them to about 400,000 years old (Meyer et al., 2014). Moreover, tests on even small fragments of bone and tooth enamel could have narrowed down the possible age range and at least ruled out either a very ancient or very young age (Grün, 2006).
As Dan Cohen points out, this discovery is also unusual in that it didn't make its splash at Science or Nature, but eLife, a free, open-access journal. Study leader Lee Berger tells Buzzfeed the article was rejected by Nature because the authors couldn't meet the mag's demands to squeeze the whole thing into 2500 words. Well, whatever the reason for going open-access, we of the science-following-but-not-institutional-password-having-audience approve.
Update: Ed Yong has a terrific account of the discovery, with a ripping narrative about the caving team's expedition and an intelligent analysis of the new species and its implications. Well worth reading.
Up until very recently, humans were thought to be the only animals who made and used stone tools, an era in human development that began roughly 3.3 million years ago. But according to this piece at the BBC, some chimpanzees and monkeys in various places around the globe have been using primitive stone tools for hundreds or even thousands of years.
Boesch and his colleagues had previously studied modern chimpanzee stone tool culture in the region. This research revealed that the chimpanzees have an idiosyncratic way of choosing and using their tools.
For instance, chimpanzees will often deliberately opt for particularly large and heavy stone hammers, between 1kg and 9kg, while humans prefer to use stones that weigh 1kg or less. Many of the 4300-year-old stone tools weighed more than 1kg, suggesting they were used by chimpanzees.
Chimpanzees also use their stone tools to crack open certain types of nuts that humans don't eat. Starch residues on some of the ancient tools came from these nuts.
Together, these findings led to an obvious conclusion: chimpanzees have been using stone tools in the rainforests of Ivory Coast for at least 4300 years.
My answer to that question, having read nothing about it beyond this article, is "it sounds like a bit of a stretch, but what an interesting thing to think about". This theory about how humans and wolves (and later, dogs) teamed up to outcompete Neanderthals for food is being forwarded by anthropologist Pat Shipman, author of the new book, The Invaders: How Humans and Their Dogs Drove Neanderthals to Extinction.
Modern humans formed an alliance with wolves soon after we entered Europe, argues Shipman. We tamed some and the dogs we bred from them were then used to chase prey and to drive off rival carnivores, including lions and leopards, that tried to steal the meat.
"Early wolf-dogs would have tracked and harassed animals like elk and bison and would have hounded them until they tired," said Shipman. "Then humans would have killed them with spears or bows and arrows.
"This meant the dogs did not need to approach these large cornered animals to finish them off -- often the most dangerous part of a hunt -- while humans didn't have to expend energy in tracking and wearing down prey. Dogs would have done that. Then we shared the meat. It was a win-win situation."
At that time, the European landscape was dominated by mammoths, rhinos, bison and several other large herbivores. Both Neanderthals and modern humans hunted them with spears and possibly bows and arrows. It would have been a tricky business made worse by competition from lions, leopards, hyenas, and other carnivores, including wolves.
"Even if you brought down a bison, within minutes other carnivores would have been lining up to attack you and steal your prey," said Shipman. The answer, she argues, was the creation of the human-wolf alliance. Previously they separately hunted the same creatures, with mixed results. Once they joined forces, they dominated the food chain in prehistoric Europe -- though this success came at a price for other species. First Neanderthals disappeared to be followed by lions, mammoths, hyenas and bison over the succeeding millennia. Humans and hunting dogs were, and still are, a deadly combination, says Shipman.
A shell found in the 1890s was recently found to have what scientists are calling the world's oldest "abstract marking", a 500,000-year-old etching made by Homo erectus, an extinct ancestor of modern humans.
Close inspection under the microscope suggested that the engraving was intentional. The weathering patterns of the grooves, each of which is about 1 centimetre long, show signs of significant ageing, and there are no gaps between turns, indicating that the maker paid attention to detail. He or she probably made the engraving on a fresh shell, and the newly made etching would have resembled white lines on a dark canvas, Joordens' team notes. Sand grains still embedded in the shell were dated to around 500,000 years ago.
A new analysis of the genomes of two extinct human species (Neanderthals and Denisovans) shows more clearly that they interbred with our species of human, contributing 2-4% of our modern genomes in some cases.
"What it begins to suggest is that we're looking at a Lord of the Rings-type world -- that there were many hominid populations," says Mark Thomas, an evolutionary geneticist at University College London who was at the meeting but was not involved in the work.
But, more interestingly, the analysis also detected the Denisovans also bred with an as-yet-unknown species of humans.
The Denisovan genome indicates that the population got around: Reich said at the meeting that as well as interbreeding with the ancestors of Oceanians, they also bred with Neanderthals and the ancestors of modern humans in China and other parts of East Asia. Most surprisingly, Reich said, the genomes indicate that Denisovans interbred with yet another extinct population of archaic humans that lived in Asia more than 30,000 years ago -- one that is neither human nor Neanderthal.
Is this the first time a new human species has been discovered through DNA evidence alone?
Trevor Paglen speculates that human civilization's longest lasting monuments will be the satellites in geostationary orbits around the Earth.
Humanity's longest lasting remnants are found among the stars. Over the last fifty years, hundreds of satellites have been launched into geosynchronous orbits, forming a ring of machines 36,000 kilometers from earth. Thousands of times further away than most other satellites, geostationary spacecraft remain locked as man-made moons in perpetual orbit long after their operational lifetimes. Geosynchronous spacecraft will be among civilization's most enduring remnants, quietly circling earth until the earth is no more.
Geoff Manaugh attended a lecture of Paglen's, where Paglen suggested a possible discoverer of these artifacts:
Billions of years from now, he began to narrate, long after city lights and the humans who made them have disappeared from the Earth, other intelligent species might eventually begin to see traces of humanity's long-since erased presence on the planet.
Consider deep-sea squid, Paglen suggested, who would have billions of years to continue developing and perfecting their incredible eyesight, a sensory skill perfect for peering through the otherwise impenetrable darkness of the oceans -- yet also an eyesight that could let them gaze out at the stars in deep space.
Perhaps, Paglen speculated, these future deep-sea squid with their extraordinary powers of sight honed precisely for focusing on tiny points of light in the darkness might drift up to the surface of the ocean on calm nights to look upward at the stars, viewing a scene that will have rearranged into whole new constellations since the last time humans walked the Earth.
And, there, the squid might notice something.
Note: Illustration by Chris Piascik...prints & more are available.
The latest word on Homo floresiensis, the potential new species of hobbit-like humans discovered ten years ago in Indonesia, concerns a pair of papers which argue the single specimen found is actually a regular human with Down syndrome.
Now, the debate has reignited with two new papers published this week by a team of researchers from Penn State and other institutions. In one of those papers, they argue that the Flores skull is not a new species, but instead represents an ancient person with Down syndrome.
The researchers also point out, in the second paper, that the original report on the bones seemed to have exaggerated the skull's diminutive size. Cranial measurements and features, along with shorter thigh bones, the team found, all correspond with modern manifestations of Down syndrome. "The difference is significant, and the revised figure falls in the range predicted for a modern human with Down syndrome from the same geographic region," they say in a statement.
Recent discoveries in Brazil that humans were present in South America as early as 22,000 years ago has contributed to the growing belief that the Clovis First hypothesis is wrong.
Researchers here say they have unearthed stone tools proving that humans reached what is now northeast Brazil as early as 22,000 years ago. Their discovery adds to the growing body of research upending a prevailing belief of 20th-century archaeology in the United States known as the Clovis model, which holds that people first arrived in the Americas from Asia about 13,000 years ago.
"If they're right, and there's a great possibility that they are, that will change everything we know about the settlement of the Americas," said Walter Neves, an evolutionary anthropologist at the University of Sao Paulo whose own analysis of an 11,000-year-old skull in Brazil implies that some ancient Americans resembled aboriginal Australians more than they did Asians.
Up and down the Americas, scholars say that the peopling of lands empty of humankind may have been far more complex than long believed. The radiocarbon dating of spear points found in the 1920s near Clovis, N.M., placed the arrival of big-game hunters across the Bering Strait about 13,000 years ago, long forming the basis of when humans were believed to have arrived in the Americas.
More recently, numerous findings have challenged that narrative. In Texas, archaeologists said in 2011 that they had found projectile points showing that hunter-gatherers had reached another site, known as Buttermilk Creek, as early as 15,500 years ago. Similarly, analysis of human DNA found at an Oregon cave determined that humans were there 14,000 years ago.
This is nutty...by chance, a group of archaeologists found what are believed to be the oldest known human footprints outside of Africa on a beach in England. The footprints are an estimated 800,000 years old and are now completely gone. The tide that uncovered them washed them away in less than a month.
The footprints have been described as "one of the most important discoveries, if not the most important discovery that has been made on [Britain's] shores," by Dr Nick Ashton of the British Museum.
"It will rewrite our understanding of the early human occupation of Britain and indeed of Europe," he told BBC News.
The markings were first indentified in May last year during a low tide. Rough seas had eroded the sandy beach to reveal a series of elongated hollows.
Old people, like those who live to be older than 30, didn't exist in great numbers until about 30,000 years ago. Why is that? Anthropologist Rachel Caspari speculates that around that time, enough people were living long enough to function as a shared cultural hard drive for humans, a living memory bank for skills, histories, family trees, etc. that helped human groups survive longer.
Caspari says it wasn't a biological change that allowed people to start living reliably to their 30s and beyond. (When she looked at other populations of Neanderthals and Homo sapiens that lived in the same place and time, the two different species had similar proportions of old people, suggesting the change was not genetic.) Instead, it was culture. Something about how people were living made it possible to survive into old age, maybe the way they found or stored food or built shelters, who knows. That's all lost-pretty much all we have of them is teeth-but once humans found a way to keep old people around, everything changed.
Old people are repositories of information, Caspari says. They know about the natural world, how to handle rare disasters, how to perform complicated skills, who is related to whom, where the food and caves and enemies are. They maintain and build intricate social networks. A lot of skills that allowed humans to take over the world take a lot of time and training to master, and they wouldn't have been perfected or passed along without old people. "They can be great teachers," Caspari says, "and they allow for more complex societies." Old people made humans human.
What's so special about age 30? That's when you're old enough to be a grandparent. Studies of modern hunter-gatherers and historical records suggest that when older people help take care of their grandchildren, the grandchildren are more likely to survive. The evolutionary advantages of living long enough to help raise our children's children may be what made it biologically plausible for us to live to once unthinkably old ages today.
Did you know that there are new human beings? Like, not just new human babies but new species of humans? And not just new species of humans, but new species of humans who lived at the same time as, and even possibly bred with, modern humans, aka us? (Helloooooo, mesofacts.)
If you've read kottke.org over the years, you've likely heard of Homo floresiensis (aka Flores man, aka Hobbits), a species whose remains were discovered in present-day Indonesia in 2003. Homo floresiensis lived 95,000 to 13,000 years ago and stood about three feet high.
In this month's National Geographic, Jamie Shreeve tells the story of the 2010 discovery of the Denisovans, hominids who lived in modern-day Russia as late as 40,000 years ago. Only a handful of bone fragments and teeth have been recovered, but DNA and other evidence suggests that Denisovans, Neanderthals, and modern humans lived together in the same place and interbred.
By the time of the Denisova symposium, Pääbo and his colleagues had published first drafts of the entire Neanderthal and Denisovan genomes. Reading so many more pages allowed Pääbo and his colleagues, including David Reich at Harvard University and Montgomery Slatkin at the University of California, Berkeley, to discover that human genomes today actually contain a small but significant amount of Neanderthal code -- on average about 2.5 percent. The Neanderthals still may have been swept into extinction by the strange, high-browed new people who followed them out of Africa, but not before some commingling that left a little Neanderthal in most of us, 50,000 years later. Only one group of modern humans escaped that influence: Africans, because the commingling happened outside that continent.
Although the Denisovans' genome showed that they were more closely related to the Neanderthals, they too had left their mark on us. But the geographic pattern of that legacy was odd. When the researchers compared the Denisovan genome with those of various modern human populations, they found no trace of it in Russia or nearby China, or anywhere else, for that matter -- except in the genomes of New Guineans, other people from islands in Melanesia, and Australian Aborigines. On average their genomes are about 5 percent Denisovan. Negritos in the Philippines have as much as 2.5 percent.
In 2012, evidence emerged of a possible new human species, the Red Deer Cave people, who lived in what is now southern China.
The Red Deer Cave dwellers' unusual features included a flat face, a broad nose, a jutting jaw that lacked a chin, large molar teeth, a rounded braincase with prominent brow ridges, and thick skull bones, the researchers say.
Their brains were "moderate in size," Curnoe added.
Despite this seemingly primitive human design, radiocarbon dating of charcoal from the fossil deposits suggests the Red Deer Cave people lived just 14,500 to 11,500 years ago, the team says-a time by which all other human species, such as Neanderthals, are thought to have died out.
As with the other potential new human species, and as is proper in science, there is some skepticism about the Red Deer Cave people.
The team's suggestion that the Red Deer Cave people are somehow evolutionarily unique is receiving a skeptical reception from other scientists.
Physical anthropologist Erik Trinkaus described the findings as "an unfortunate overinterpretation and misinterpretation of robust early modern humans, probably with affinities to modern Melanesians"-indigenous peoples of Pacific islands stretching from New Guinea to Fiji (map).
"There is nothing extraordinary" about the newly announced fossil human, added Trinkaus, of Washington University in St. Louis, via email.
Philipp Gunz, of Max Planck Institute for Evolutionary Anthropology in Germany, isn't convinced by the study team's interpretation either.
"I would be surprised if it really was a new human group that was previously undiscovered," said, Gunz, also a physical anthropologist.
In the last few years, scientists have discovered that before Neanderthals went extinct around 30,000 years ago, they interbred with modern humans. As a result, many humans alive today contain Neanderthal DNA in their genomes, typically between 1-4%.
Yesterday, a few of the editors at The Atlantic had their genes analyzed for Neanderthal DNA: Alexis Madrigal had 3.6%, Steve Clemons had 4.3%, and James Fallows had 5%. Personal genetic information company 23andMe added the ability to determine your Neanderthal DNA percentage a few months ago and it turns out 2.7% of my DNA is from Neanderthals, compared to 2.5% for the average 23andMe user.
If you have a 23andMe acct, you can check your percentage by logging in and going to "Ancestry Labs" in the sidebar.
Elephants were the "perfect food package" for pre-human hominids (they were slow with a good fat-to-protein ratio) and their extinction in the Middle East (and Africa) caused those hominids to evolve to be more like modern humans.
When elephants began to die out, Homo erectus "needed to hunt many smaller, more evasive animals. Energy requirements increased, but with plant and protein intake limited, the source had to come from fat. He had to become calculated about hunting," Ben-Dor says, noting that this change is evident in the physical appearance of modern humans, lighter than Homo erectus and with larger brains.
Whoa, watch the video at the top of this article to see how the human face develops in the womb from an age of one-month to ten weeks. It all just comes together right at the end!
If you watch it closely, you will see that the human face is actually formed of three main sections which rotate and come together in an unborn foetus.
The way this happens only really makes sense when you realise that, strange though it may sound, we are actually descended from fish.
The early human embryo looks very similar to the embryo of any other mammal, bird or amphibian -- all of which have evolved from fish.
Your eyes start out on the sides of your head, but then move to the middle.
The top lip along with the jaw and palate started life as gill-like structures on your neck. Your nostrils and the middle part of your lip come down from the top of your head.
There is no trace of a scar; the plates of tissue and muscle fuse seamlessly. But there is, however, a little remnant of all this activity in the middle of your top lip -- your philtrum.
The discovery of stone tools that are possibly 130,000 years old on the island of Crete may indicate that humans were seafaring far earlier than is commonly believed.
Archaeologists and experts on early nautical history said the discovery appeared to show that these surprisingly ancient mariners had craft sturdier and more reliable than rafts. They also must have had the cognitive ability to conceive and carry out repeated water crossing over great distances in order to establish sustainable populations producing an abundance of stone artifacts.
Internet, meet Ardi, the newest member of the human branch of the primate family tree.
Or rather, the oldest. Discovered in Ethiopia in 1994, Ardi is a 4.4 million-year-old partial skeleton of a female Ardipithecus ramidus.
The fossil puts to rest the notion, popular since Darwin's time, that a chimpanzee-like missing link -- resembling something between humans and today's apes -- would eventually be found at the root of the human family tree. Indeed, the new evidence suggests that the study of chimpanzee anatomy and behavior -- long used to infer the nature of the earliest human ancestors -- is largely irrelevant to understanding our beginnings.
Ardi instead shows an unexpected mix of advanced characteristics and of primitive traits seen in much older apes that were unlike chimps or gorillas. As such, the skeleton offers a window on what the last common ancestor of humans and living apes might have been like.
This is a major discovery; Science is devoting a special issue to the find with 11 detailed peer-review papers and general summaries. I expect we'll be hearing more about this in the coming weeks as all that science filters through the lay media. (thx, jeff)
Some poop in a cave in Oregon has been dated to more than 14,000 years ago and identified as human, adding to other evidence that humans inhabited the Americas before the well-known Clovis people.
Other archaeologists agreed that the findings established more firmly than before the presence of people on the continent at least 1,000 years before the well-known Clovis people, previously thought to be the first Americans. Recent research at sites in Florida and Wisconsin also appears to support the earlier arrivals, and a campsite in Chile indicates migration deep into South America by 14,600 years ago.
Speaking of memes, Susan Blackmore theorizes that humans are just machines for propagating them.
Memes are using human brains as their copying machinery. So we need to understand the way human beings work.
Up until very recently in the world of memes, humans did all the varying and selecting. We had machines that copied -- photocopiers, printing presses -- but only very recently do we have artificial machines that also produce the variations, for example (software that) mixes up ideas and produces an essay or neural networks that produce new music and do the selecting. There are machines that will choose which music you listen to. It's all shifting that way because evolution by natural selection is inevitable. There's a shift to the machines doing all of that.
When asked what the future will look like, she says, "it will look like humans are just a minor thing on this planet with masses (of) silicon-based machinery using us to drag stuff out of the ground to build more machines."
The pace of human evolution has accelerated greatly over the last 40,000 years, partially due to our population growth.
The brisk rate of human selection occurred for two reasons, Dr. Moyzis' team says. One was that the population started to grow, first in Africa and then in the rest of the world after the first modern humans left Africa. The larger size of the population meant that there were more mutations for natural selection to work on. The second reason for the accelerated evolution was that the expanding human populations in Africa and Eurasia were encountering climates and diseases to which they had to adapt genetically. The extra mutations in their growing populations allowed them to do so.
Dr. Moyzis said it was widely assumed that once people developed culture, they protected themselves from the environment and from the forces of natural selection. But people also had to adapt to the environments that their culture created, and the new analysis shows that evolution continued even faster than before.
Looks like this study answers the "Is Evolution Over?" question.
A timeline of human history (mostly sex and violence) by Milo Manara. NSFW.
Some recent research on the wrist bones of the so-called hobbit skeleton suggests that Flores man is an ancestor of modern humans and not just diseased homo sapiens. The debate continues. (via npr)
Humans are the animal world's best distance runners...we can run long distances relatively fast without overheating. "Once humans start running, it only takes a bit more energy for us to run faster, Lieberman said. Other animals, on the other hand, expend a lot more energy as they speed up, particularly when they switch from a trot to a gallop, which most animals cannot maintain over long distances." (via beebo)
I thought that said "Netherlanders"...I was ready to put that in the "odd things I didn't know about the Dutch" column.
An evolutionary theorist has predicted that humans may split into two sub-species: "the genetic upper class would be tall, slim, healthy, attractive, intelligent, and creative and a far cry from the 'underclass' humans who would have evolved into dim-witted, ugly, squat goblin-like creatures."
Watched America's Stone Age Explorers on PBS this evening, a summary of recent findings about who the first Americans were, where they came from, and when they arrived. Recent genetic and archeological evidence suggests they arrived earlier than generally accepted and may have originated from Europe rather than Asia.
Recently discovered human remains suggest that metrosexuals lived in Iron Age Ireland. One man's fingernails were manicured and his hands indicated he'd never done manual labor while the other wore hair gel imported from France and Spain. No word on if they wore their shirts tucked or untucked.
Further discovery of Homo floresiensis bones have strengthened researchers' argument that the so-called Hobbit is a new and distinct human species. More on Flores man at Nature, which is doing a weekly podcast now.
A US antropologist says that weaker toes found in human skeletons from 26,000-40,000 years ago indicates when humans started wearing sturdy shoes.
Robotics research suggests that Lucy walked upright like humans. Lucy, discovered in 1974 in Ethiopia, is a 3.2 million year old Australopithecus afarensis skeleton.
40,000 year-old human footprints found in Mexico. Humans are thought to have come to the Americas only 11,000 years ago.