April is Citizen Science Month! In this post, graduate student Mandy Dees illustrates the work of Dr. Gabriel Filipelli and how research…
Here we are, once again, at the end of a calendar year filled with lots of exciting news in the field of human evolution. Last year, just as we were finalizing edits on the 2017 Top 5 Human Evolution Discoveries list, the remainder of the skeleton of a human ancestor known colloquially as “Little Foot” (belonging to the genus Australopithecus, the same genus, but different species, as the famed “Lucy” fossil) was finally revealed after 20 years of cleaning and excavation from its embedding rock. Amazingly, just as we are finishing the edits for this year’s installment of top human evolution discoveries, Little Foot is back in the news. As of the last week of November, full descriptions and analyses of the remainder of the fossils are now available (prior to undergoing peer-review) on the preprint server bioRxiv. Enjoy reading our Top 6 list for 2018! Why 6? These stories are too cool not to share.–JMO
What does it mean to be human? What makes us unique among all other organisms on Earth? Is it cooperation? Conflict? Creativity? Cognition? There happens to be one anatomical feature that distinguishes modern humans (Homo sapiens) from every other living and extinct animal: our bony chin! But does a feature of our jaws have actual meaning for our humanity? We want to talk about the top six discoveries of 2018, all from the last 500,000 years of human evolution, that give us more insight into what it means to be human. If you want to learn more about our favorite discoveries from last year, read our 2017 blog post!
1) Migrating modern humans: the oldest modern human fossil found outside of Africa
Every person alive on the planet today is a Homo sapiens,and our species evolved around 300,000 years ago in Africa. In January of this year, a team of archaeologists led by Israel Hershkovitz from Tel Aviv University in Israel made a stunning discovery at a site on the western slope of Mount Carmel in Israel—Misliya Cave. This site had previously yielded flint artifacts dated to between 140,000 and 250,000 years ago, and the assumption was that these tools were made by Neanderthals which had also occupied Israel at this time. But tucked in the same layer of sediment as the stone tools was a Homo sapiens upper jaw! Dated to between 177,000 and 194,000 years ago by three different dating techniques, this finding pushes back the evidence for human expansion out of Africa by roughly 40,000 years. It also supports the idea that there were multiple waves of modern humans migrating out of Africa during this time, some of which may not have survived to pass on their genes to modern humans alive today. Remarkably, this jawbone was discovered by a freshman student at Tel Aviv University working on his first archaeological dig in 2002! So, there is hope for students wishing to make a splash in this field!
2) Innovating modern humans: long-distance trade, the use of color, and the oldest Middle Stone Age tools in Africa
At the prehistoric site of Olorgesailie in southern Kenya, years of careful climate research and meticulous excavation by a research team lead by Rick Potts of the Smithsonian’s National Museum of Natural History and Alison Brooks of George Washington University were able to explore both the archaeological and paleoenvironmental records to document behavioral change by modern humans in response to climatic variation. The artifacts show a shift from the larger and clunkier tools of the Acheulean, characterized by teardrop-shaped handaxes, to the more sophisticated and specialized tools of the Middle Stone Age (MSA). The MSA tools were dated to 320,000 years ago, the earliest evidence of this kind of technology in Africa. They also found evidence that one of the kinds of rock used to make the MSA tools, obsidian, was obtained from at least 55 miles (95 kilometers) away. Such long distances led the teams to conclude that obsidian was traded in social networks, since this is much further than modern human forager groups typically travel in a day. On top of that, the team found red and black rocks (pigments) used for coloring material in the MSA sites, indicating symbolic communication, possibly used to maintain these social networks with distant groups. Finally, all of these innovations occurred during a time of great climate and landscape instability and unpredictability, with a major change in mammal species (about 85%). In the face of this uncertainty, early members of our species seem to have responded by developing technological innovations, greater social connections, and symbolic communication. These exciting findings were published in a set of three papers in Science, focused on the dating of these finds; the stone tool technology and transport and use of pigments; and the earlier changes in environments and technology that anticipate later characteristics of the stone tools.
(Featured image at the top of this post is the famous “Catwalk Site”, one of the open air displays at the National Museums of Kenya Olorgesailie site museum, which is littered with ~900,000 year old handaxes. Photo courtesy of Briana Pobiner.)
3) Art-making Neanderthals: our close evolutionary cousins actually created the oldest known cave paintings
Neanderthals are often imagined as primitive brutes dragging clubs behind them. But new discoveries, including one made last year, continue to reshape that image. A team led by Alistair Pike from the University of Southampton found red ocher paintings– dots, boxes, abstract animal figures, and handprints – deep inside three Spanish caves. The most amazing part? These paintings dated to at least 65,000 years ago–a full 20,000-25,000 years before Homo sapiens arrived in Europe (which was 40,000 to 45,000 years ago)! The age of the paintings was determined by using uranium-thorium dating of white crusts made of calcium carbonate that had formed on top of the paintings by water percolating through the rocks. Since the calcite precipitated on top of the paintings, the paintings must have been there first – so they are older than the age of the calcite. The age of the paintings suggests that Neanderthals made them. It has been generally assumed that symbolic thought (the representation of reality through abstract concepts, such as art) was a uniquely Homo sapiensability. But sharing our ability for symbolic thought with Neanderthals means we may have to redraw our images of Neanderthal in popular culture: forget the club, maybe they should be holding paint brushes instead.
4) Trekking modern humans: the oldest modern human footprints in North America
When we think about how we make our marks on this world, we often picture leaving behind cave paintings, structures, old fire pits, and discarded objects. But even a footprint can leave behind traces of past movement! A discovery this year by a team led by Duncan McLaran from the University of Victoria with representatives from the Heiltsuk and Wuikinuxv First Nations revealed the oldest footprints in North America! These 29 footprints were made by at least three people on the tiny Canadian island of Calvert. The team used Carbon-14 dating of fossilized wood found in association with the footprints to date the find to 13,000 years ago. This site may have been a stop on a late Pleistocene coastal route humans used when migrating from Asia to the Americas. Because of their small size, some of the footprints must have been made by a child – who would have worn about a size 7 kids shoe today, if they were wearing shoes (interestingly, the evidence indicates they were walking barefoot). As humans, our social and caregiving nature has been essential to our survival. One of the research team members, Jennifer Walkus, mentioned why the child’s footprints were particularly special: “Because so often kids are absent from the archeological record. This really makes the archaeology more personal.” Any site with preserved human footprints is pretty special, as there are currently only a few dozen in the world.
5) Winter-stressed, nursing Neanderthals: Neanderthal children’s teeth reveal intimate details of their daily lives
Evidence of children is very rare in the prehistoric archaeological record; their bones are more delicate than those of adults and therefore less likely to survive and fossilize, and their material artifacts are also almost impossible to identify. For instance, a stone tool made by a child might be interpreted as made hastily or by a novice, and toys are quite a new invention. To find remains that are conclusively juvenile is very exciting to archaeologists – not only for the personal connection we feel, but for the new insights we can learn about how individuals grew, flourished, and according to a new study led by Dr. Tanya Smith from Griffith University in Australia, suffered. Smith and her team studied the teeth of two Neanderthal children who lived 250,000 years ago in southern France. They took thin sections of the two teeth and “read” the layers of enamel, which develops in a way similar to tree rings: in times of stress, slight variations occur in the layers of tooth enamel. The tooth enamel chemistry also recorded environmental variation based on the climate where the Neanderthals grew up, because it reflects the chemistry of the water and the food that the Neanderthals kids ate and drank. The team determined that the two young Neanderthals were physically stressed during the winter months – they likely experienced fevers, vitamin deficiency, or disease more often during the colder seasons. The team found repeated high levels of lead exposure in both Neanderthal teeth, though the exact source of the lead is unclear – it could have been from eating or drinking contaminated food or water, or inhaling smoke from a fire made from contaminated material. They also found that one of the Neanderthals was born in the spring and weaned in the fall, and nursed until it was about 2.5 years old, similar to the average age of weaning in non-industrial modern human populations. (Our closest living relatives (chimpanzees and bonobos) nurse for much longer than we do, up to 5 years.) Discoveries like this are another indication that Neanderthals are more similar to Homo sapiensthan we had ever thought. Paleoanthropologist Kristin Krueger notes how discoveries like this are making “the dividing line between ‘them’ and ‘us’ [become more blurry] every day.”
6) Hybridizing hominins: the first discovery of an ancient human hybrid
Speaking of blurring lines (and probably the biggest story of the year): a new discovery from Denisova Cave in Siberia has added to the complicated history of Neanderthals and other ancient human species. While Neanderthal fossils have been known for nearly two centuries, Denisovans are a population of hominins only discovered in 2008 based on the sequencing of their genome from a 41,000-year-old finger bone fragment from Denisova Cave – which was also inhabited by Neanderthals and modern humans (and whom they also mated with). While all of the known Denisovan fossils could nearly fit in one of your hands, the amount of information we can gain from their DNA is enormous! This year, a stunning discovery was made from a fragment of a long bone identified as coming from a 13-year-old girl nicknamed “Denny” who lived about 90,000 years ago: she was the daughter of a Neanderthal mother and Denisovan father. A team led by Viviane Slon and Svante Pääbo from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany first looked at her mitochondrial DNA and found that it was Neanderthal – but that didn’t seem to be her whole genetic story. They then sequenced her nuclear genome and compared it to the genomes of other Neanderthals and Denisovans from the same cave, and compared it to a modern human with no Neanderthal ancestry. They found that about 40% of Denny’s DNA fragments matched a Neanderthal genome, and another 40% matched a Denisovan genome. The team then realized that this meant she had acquired one set of chromosomes from each of her parents, who must have been two different types of early humans. Since her mitochondrial DNA – which is inherited from your mother – was Neanderthal, the team could say with certainty that her mother was a Neanderthal and a father that was Denisovan. However, the research team is very careful about not using the word “hybrid” in their paper, instead stating instead that Denny is a “first generation person of mixed ancestry.” They note the tenuous nature of the biological species concept: the idea that one major way to distinguish one species from another is that individuals of different species cannot mate and produce fertile offspring. Yet we see interbreeding commonly occurring in the natural world, especially when two populations seem to be in the early stages of speciating – because speciation is a process that often takes a long time. It is clear from genetic evidence that Neanderthals and Homo sapiens individuals were sometimes able to mate and produce children, but it is unclear if these matings included difficulty with becoming pregnant or bringing a fetus to term – and modern human females and Neanderthal males may have had particular trouble making babies. While Neanderthals contributed DNA to the modern human genome, the reverse seems not to have occurred. Regardless of the complicated history of intermingling of different early human groups, Dr. Skoglund from the Francis Crick institute echoes what many other researchers are thinking about this amazing discovery, “[that Denny might be] the most fascinating person who has had their genome sequenced.”
Edited by Jason Organ, PhD, Indiana University School of Medicine.