How Do We Know Who Our Human Ancestors Were?

Whenever bones that look like ours are discovered,
paleoanthropologists are on their knees looking for the answer. Are they human, or are they
our ancestors? Hello you bunch of homo sapiens, Julian here
for DNews. Recently scientists announced a huge find, 15 partial human-like skeletons
in a cave in South Africa. The skeletons belong to a species no one has seen before, and so
a new species was added to the human evolutionary tree: Homo naledi. Homo is a really cool word, because it pulls
double duty. From its greek origins, it means “the same,” and is the opposite of “hetero.”
The latin root of homo means “man” or “human being.” When we find a new species
and put in the genus Homo, it’s a big deal. But how do we figure out what species deserves
the honor of being called Homo? Well the skull is a good place to start. Humans
today have large brains and small teeth. Our cousins, the apes, are the opposite. So, when
figuring out if a skull belongs to your family tree, we look there. Mind you, just because
a creature had small teeth doesn’t mean it had a large brain. Homo naledi is a good
example of that. The teeth are small and primitive, but the brain is only about as big as a large
orange. Homo naledi is about 3 million years old though, and from the fossil record right
now we believe that brain size really started ramping up 800,000 to 200,000 years ago. And
I can already hear a lot of you asking me about dating, and I’d love to. Oh you meant the age of the skeletons?…
Alright, that’s fine, I’m not heartbroken or anything.
There are actually multiple accurate methods used for figuring out how old a fossil is
and thus where it fits on the human timeline. For younger fossils with DNA still intact,
we can compare how similar the DNA is to ours and other species. Older fossils can be dated
by examining how much of certain radioactive elements are present. These elements, like
potassium, argon, carbon 14, and uranium decay at a steady rate, so measuring how much is
left can accurately place a date. There’s thermo-luminescence, optically stimulated
luminescence, and electron spin resonance methods that measure how many electrons have
been trapped in a rock or tooth over eons. What I’m getting at is we don’t just find
bones and go, “Gee that looks super old, I’m gonna go with 3 million years. Yep,
that sounds about right.” Fossils are finicky things though, what with
being millions of years old and often buried accidentally, and usually a lot of the pieces
are gone. But even without a skull the other bones can tell us how close a species was
to human. The shape of the spine, legs, hips, and feet can tell us if a species walked upright,
lived in trees, or some combination of both. How thick the bones are can tell us how active
they were. Teeth and gut size can tell us what they ate, with larger guts being useful
for digesting plants. The hands can either be flat like ours or curved like apes. And
finally, where the bones are found and other items around them give us clues about our
ancestors. Did they have tools? Did they have language and culture? Homo naledi was a big
surprise because even though the fossils are so old, they appeared to be buried in the
cave like a burial chamber. We thought that was a much more recent human habit, so our
understanding will have to be reshaped. When talking about the remains of those that
came before us, it helps to remember that they don’t all fall neatly in a straight
line. As we were finding our way into existence, other species branched off and became more
different, like Homo floresiensis, a recent relative who became tiny with smaller brains
and larger teeth. Others branched out and then mixed back in with our ancestors a bit,
like the now-extinct neanderthals. Instead of a simple line, human lineage is more like
a tree, Below the Homo branch of the tree there’s the Paranthropus group, and another
separate branch belongs to the Australopithecus. Farthest down and closest to where our line
split from apes is the Ardipithecus branch. Finding a fossil takes a lot of luck, and
determining where each fossil fits on the path to become us is still debated by paleoanthropologists.
Homo naledi is up for that same rigorous debate, and where it fits in the puzzle may change.
But right now the clues the skull, skeleton, age, and environment make it the oldest member
of the Homo genus yet. For me, looking across the millions of years of trial and error it
took for us to finally exist gives me a great appreciation for being human. For a more in depth look of how we got here,
Trace tackles that on Test Tube Plus right here.


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