A few days ago my three-year-old asked me, “How do humans get on our planet?” Preschool grammar aside, I was impressed by her question, and so, being an astronomer, I launched into an excited explanation of nucleosynthesis in the cores of stars and the creation of the heavier elements. As Carl Sagan reminds us, “We are made of star stuff.” This was not the answer she was looking for.
Luckily, before I had to attempt an explanation of the primordial soup fit for a three-year-old, she interrupted me to ask instead, “Can we learn about people from their bones like the dinosaurs?” I told her that not only is that the case, but we continue to learn about our origins as more bones are discovered and as our dating techniques improve. In fact, a study published just last month in the journal Nature challenges what we thought we knew about human origins and migration.
How do we date human bones?
The challenge in identifying the oldest human remains is two-fold: first, the bones need to be dated correctly. Scientists use several methods for determining the ages of human remains and those methods can often be checked against one another for consistency.
Age-dating via electron spin resonance, for example, measures how many electrons have been absorbed by the bone over time. Radioactive elements decay over time on a precisely-timed schedule, and so other methods like potassium-argon dating or Carbon-14 dating, measure the extent to which these elements have decayed to assess how long the bone has been waiting to be unearthed. A technique called paleomagnetism examines the direction of the magnetized particles in nearby rock and links it to known global shifts in our planet’s magnetic field.
Scientists also use context to help place age constraints – were any artifacts or tools found nearby? Where were the bones found? While context can be extremely helpful, particularly in cases where only a few bones are found, it also must be used with caution so as not to preclude new ways of thinking about our origins.
Another challenge in identifying the oldest human skeleton is determining whether or not new bone discoveries are in fact, human. Neanderthals, for example, who we once thought were our ancestors but now believe were a kind of distant cousin with a shared ancestor, have longer and flatter skulls. It is hypothesized that our facial features shortened as our brain organization and connectivity improved. So while there are key markers that distinguish a member of the homo sapien species from other hominin relatives, incomplete fossils and evolutionary changes that have led to what we recognize now as modern day humans can make the identification of these markers challenging.
The Oldest Fossil Bones
The skeleton known as Lucy is often credited as being the oldest known human relative at an estimated age of 3.2 million years. She is thought to be of the hominin species Australopithecus, an ancestor to modern humans. Another fossil found in Ethiopia two years ago, only a dozen miles from Lucy’s resting place, is believed to be more directly linked to modern humans, and would thus push our lineage back another half a million years to 2.8 million years ago. 90,000 year old bones believed to be human have been found in Saudi Arabia. Some of the oldest human bones found in the Americas, a complete 13,000-year-old skeleton, was discovered in a 140-foot deep cave buried underwater near Mexico’s Yucatan Peninsula.
New Discoveries in Human Migration
We have known from fossils for some time that we, homo sapiens, originated in Africa. There have been so many fossil remains discovered in East Africa that the area has been dubbed the cradle of humankind and the Garden of Eden.
A map published in Nature just last year draws out the route for homo sapiens migrating from East Africa 150,000 to 200,000 years ago and being found in the Arabian peninsula as long as 120,000 years ago. The histories of homo sapiens in Europe and Australia are much younger (dating back as far as 45,000 years ago and 50,000 years ago, respectively) but still longer than in the Americas and North Asia (15,000 to 20,000 years in the past).
A new discovery published last month in the journal Nature suggests, however, that we may have to expand what we know about humanity’s cradle. The labrynth of caves in Jebel Irhoud, not far from Marrakesh in Morocco, has been of interest to anthropologists since miners found bones, including a hominid skull in the 1960s. The skull was thought to be a Neanderthal and only dated at 40,000 years old. After additional bones were found, including a child’s jawbone, with questionable dates, the authors of the new study, led by Jean-Jacques Hublin of the Max Planck Institute for Evolutionary Anthropology, began a more thorough excavation of the area in 2004.
Their thorough exploration, together with improved age-dating techniques, revealed that the bones discovered there now, thought to be early humans, date back as long as 300,000 years ago. Finding these old bones so far outside of East Africa suggests that all of Africa may in fact be the figurative Garden of Eden.Scientists believed East Africa to be the cradle of humanity because so many fossilized skeletons were found there. But we also found so many fossilized skeletons there because that is where we were looking. Astronomers run into similar observational biases when trying to track down our origins. For example, the first extrasolar planets found, planets outside of our solar system, were more Jupiter-like than Earth-like, but that was only because our techniques for finding them were better at picking out Jupiters.