I love a good “best of” list at the end of each out-going year. A study by scientists who ate Legos and studied their “passing” made one such list for 2018 (Everything is Awesome: Don’t Forget the Lego). To the Google for more info on that one. My favorite top science story for the year was the story of Denny, a child from a very surprising blended family.
A very tangled family tree
Have you seen this poster?
The March of Progress, or The Road to Homo Sapiens was originally published by Time-Life in 1965.
It has been reprinted and repeated, held up as science fact and disparaged as the march to Godless evolution.
Both interpretations are wrong.
In reality, the Road to Homo Sapiens is no more scientifically correct than this version:
The evolution of modern humans was not an all-in-line march to the finish as the famous March to Progress illustration implies.
Modern humans sit on one tip of a branch of an ancient human family tree – a tangled tree with many branches.
All of the other branches in this tangled tree have died out. We alone survive.
But in the not-so-far past (relatively speaking), this was not the case. In the past, modern humans shared our planet with some of the now extinct branches of our tangled family tree.
Last to leave, last to arrive
Modern humans have lived longer in Africa than any other place on earth – about 200,000 years.
Many modern humans stayed in Africa. Other groups of modern humans left Africa about 70,000 years ago and spread across Europe, Asia, the Pacific, and the Americas (see map).
But modern humans were late to the out-of-Africa party.
An ancestor of modern humans had already ventured out of Africa 500,000 – 600,000 years before. Once out of Africa, this ancestor group further split into at least two important groups: the Neanderthals and the Denisovans. The members of the ancestral group that remained in Africa gave rise to modern humans.
The Neanderthals spread out across Europe and western Asia and the Denisovans ranged from eastern Europe to eastern Asia. When modern humans finally moved out of Africa and trekked across the globe, they met some very ancient cousins. But by 30,000 years ago, modern humans stood alone – the last remaining branch on the tangled human family tree.
All three human groups (Neanderthals, Denisovans, modern humans) are distinct – some scientists consider them different species. However, modern humans were closely related enough to mate and have children with the other two groups.
How do we know this? People with European and Asian ancestry have trace amounts (one to two percent) Neanderthal DNA in their genomes. Denisovan DNA is highest in the modern populations of southeast Asia and Oceania (4 – 6%). Trace amounts of Denisovan DNA are also found in east Asian populations. Interestingly, people in sub-Saharan Africa have zero-to-almost zero Neanderthal or Denisovan DNA.
An extraordinary new discovery tells us that early human groups also mated with each other.
For the last twelve years, a single cave in Siberia has produced important discoveries of Denisovan remains. The Denisova cave (for which this group was named) yielded an astonishing new find, first announced in August 2018.
A bone from a thirteen-year-old girl was found with equal amounts of Denisovan and Neanderthal DNA.
Humans have 23 unique chromosomes. But – we have two copies of each chromosome, one from biological mom and one from biological dad.
The chromosomes of the 13 -year old girl found in the Denisovan cave (nicknamed “Denny”) stunned scientists. In each chromosome pair, one chromosome came from a parent that was exclusively Neanderthal and one came from a parent that was exclusively Denisovan.
Additionally, humans have a tiny bit of DNA in the mitochondria of their cells. All mitochondrial DNA comes from mom (I wrote about mitochondrial DNA in this post). Denny’s mitochondrial DNA was Neanderthal.
It was as if we had a front-row seat. Denny was a first-generation offspring of a Neanderthal mom and a Denisovan dad.
The first reaction of the researcher studying Denny’s DNA was “what did I do wrong?”. The tests were repeated six times, and each time the results were the same.
Denny is an exciting, but not surprising find. We have indirect evidence of interbreeding: trace amounts of DNA from “cousin” human groups have been found in Neanderthals, Denisovans, and modern humans. With the discovery of Denny, we have direct evidence of interbreeding between human groups. How often did that happen? That’s a question to be answered, but Denny provides a hint. We’ve only known about Denisovans since 2008, and already we have a first-generation hybrid with another human group. We have yet to find a first-generation offspring of modern humans and Neanderthals or Denisovans, but they surely existed.
Do you have more questions about the tangled human family tree? Here’s a link to an excellent six-minute animated graphic Last Hominin Standing: Charting Our Rise and the Fall of Our Closest Relatives.
And as You speak
A hundred billion creatures catch Your breath
Evolving in pursuit of what You said
If it all reveals Your nature so will I
(Hillsong United “So Will I“)