👦🏽👩🏿🦱👱♀️🧔🏾🧒🏼👩🦲👨🏻👵🏽🧑🏻👴👶🏿👧🏾
A long time ago, your lineage looked more like this:🐵🐵🙈🐵🐵🙉🐵🐵🐵🙊🐵🐵
What happened? Individuals vary within a species, and the sorting out of those variants over time causes a species to change. That’s evolution. It’s a simple concept. But these two ideas – that humans are different from each other, and that humans evolved – can make us uncomfortable. Many folks mistrust the science, misunderstand the science, or misconstrue the science. This guide will try to clear everything up.🙈
Don’t be this guy
Humans variation is correlated with geography. People living in one place might look like this:
👩🏾🎨👨🏿💻👩🏿🍳👨🏿🏭👨🏾🎤👨🏾🎓
People from Site 1
And people from another place might look like this:
👩🏻🚒👮🏻♀️👩🏻🔬👨🏼🚀👩🏼🌾👨🏼⚖️
People from Site 2
How to make sense of the differences? A common strategy is to divide people into “races”. But “race” isn’t a biology word and isn’t defined by what biologists can measure. The idea of race is tied up with culture and social assumptions and isn’t particularly well correlated with actual genetics.
Alan👷🏿♂️
Bob👨🏼🔧
Chuck👨🏿🏭
Alan is genetically more similar to Bob than to Chuck, but Alan and Chuck are the same race and Bob is another race
That’s why geneticists use words like “ancestry” and “population” rather than “race.” Those terms aren’t just synonyms for “race,” which is largely defined by subjective beliefs and social norms. Instead, they’re defined by objective data like who your close relatives are and where in the world they live. But even that can cause confusion. A common mistake is to think of human populations as discrete units, like islands in the sea.
🌊🌊🏝️🌊🌊🌊🌊🌊🏝️🌊🌊
Population 1 Population 2
Instead, it’s more realistic to think of each person as an island, and populations as archipelagos. We can find clusters of islands and call them distinct archipelagos, but it’s somewhat subjective and arbitrary. Like this:
🏝️🌊🏝️🌊🏝️🌊🌊🌊🏝️🌊🏝️🌊🌊🏝️🌊🏝️
🟥🟥 1 🟥🟥🟥🟥🟨🟨🟨🟨 2 🟨🟨🟨🟨
Or alternatively:🏝️🌊🏝️🌊🏝️🌊🌊🌊🏝️🌊🏝️🌊🌊🏝️🌊🏝️
🟥🟥 1 🟥🟥🟥🟨🟨🟨 2 🟨🟨🟪🟪 3 🟪
Even accepting that the borders between populations are fuzzy, there is a tendency to overestimate the differences. It’s true that humans across the globe have been subject to slightly different evolutionary pressures and random changes, leading to distinct genetic characteristics. And it’s true that these are the same kind of adaptive processes that can eventually cause species to diverge. So far, so good. But at this point it’s easy to be misled. To see why, consider the big picture of human evolution. We’ve all seen those diagrams that show a parade of progress, from curly-tailed monkey, to knuckle-dragging ape, to hairy caveman, to erect modern human.🐒 →🦍 →🧔🏽 →👨💻
A common, but misleading, depiction of evolution
There are a few problems with such images, such as:
👩💻❓
Where are the women?
and
🐒🐒🐒🐒🐒🐒 →🦍🦍🦍🦍🦍🦍
Populations, not individuals, evolve
and
🧔🏽👩🏽🦱👨🏽🦱🧑🏽👴🏽👨🏽🦲 🦍🦍🦍🦍🦍🦍
⬉⬈
🐒🐒🐒🐒🐒🐒
Populations are continuously-evolving branches, not static steps in a linear progression
and
👩🏽💻👨💻🧑🏾💻🧑🏻💻👨🏿💻👩💻 🦍🦍🦍🦍🦍🦍
↑ ↑
🧔🏽👩🏽🦱👨🏽🦱🧑🏽👴🏽👨🏽🦲 🦍🦍🦍🦍🦍🦍
⬉⬈
🐒🐒🐒🐒🐒🐒
All living humans, and all other living organisms, are equally “modern”
Also, focus on the space between each advancing primate.
🐒 →🦍 →🧔🏽 →👨💻
Here it is again
You might assume from the picture that the amount of change between apes and ancient humans should be similar to the amount of change between ancient humans and today. However, a little math will show how wrong this assumption is.
🧮
Get out your calculator
The last common ancestor of all living humans is estimated to have lived only three or four thousand years ago(Rohde et al. 2004). That was about 120 generations ago. Of course, at that time there were already humans living in very different cultures all over the world. But there were always a few travelers, even across continents, sowing their wild oats.
Number of “greats” (generations) for who shares a great-great-etc. grandparent with you:
100👨🏽👩🏽
101🧑🏾🧓🏽
102🧔🏼👩🏿🦱(all humans)
103
104
105
106🦍🦧(all apes)
107🐵🐒
108🐰🐯
109🦈🐙(all animals)
1010🌼🌲
1011
1012🦠🌏(all life)
Picture 120 people standing in a line. The first is your mother or father, the next is one of your grandparents, then a great-grandparent, and so on. They would take up about the length of a football field.🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂🙂
=🏟️
120 ancestors = 1 football field
Every person on Earth today could trace a line back to the same ancestor at the end zone. We’re all family, 120th cousins or closer. However, that greatest grandparent didn’t actually impart DNA to all of us. Chromosomes get shuffled with every new baby. Each parent only provides about half of your DNA, and so eventually the traces of most ancestors are lost.
👨🏻🦰×👩🏻🦱
↓
👩🏻🦰
The DNA behind Mom’s hair might have missed you
To find someone who actually contributed genetically to all modern humans, we’d need to go back to the dawn of modern humans, between 100 and 200 thousand years ago. About 6000 generations. In other words, a queue of ancestors the length of fifty football fields laid end to end, reaching just under three miles. A bit longer than Central Park. You could stroll past them in less than an hour.
🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️🏟️
= 🚶🏽♂️
50 football fields = 1 walk
All of these 6000 ancestors are completely human, physically indistinguishable from folks alive today. To escape our species, you’d need to go back farther. Our closest living relatives are the chimpanzees, which shared an ancestor with us around six million years ago. That’s about 300,000 generations. The line now stretches over 140 miles. The width of Indiana. A trek composed of fifty park-sized walks, or a highway drive of a couple of hours.
🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️🚶🏽♂️
=🚗
50 walks = 1 drive
As you drove past them, you ancestors would gradually look less human, but change would be slow. To get to actual monkeys, the kind with tails, the line would need to be even longer. Our common ancestor with baboons would stand behind more than a million other ancestors, 800 miles behind the start of the line. The distance from Canada to the Gulf of Mexico. Our ancestor with other monkeys like capuchins would need an even longer line. If you wanted to pass by all of them in less than a day, you’d want an airplane.
🚗🚗🚗🚗🚗… =✈️
5 (or more) drives = 1 flight
The point is that humans haven’t been separated for very long. Evolution is slow. 1500 BC sounds like a long time ago, but it’s a blip compared to Earth’s prehistory. The amount of evolutionary change that has happened within the human species is a blip compared to the amount of change between species. So while you could think of the gaps among human populations as just “smaller versions” of the gaps between species, you need to remember that they are a lot smaller:
🐒→→[insert thousands of arrows here]→→👨🏼🔧👨🏿🏭
This is not to say that big evolutionary changes can’t happen on short time scales. Just look at agriculture. Most of our food comes from organisms that we have bred to be dramatically different from the way their wild ancestors were just a few thousand years ago:🥔toxic
🌽tiny
🥕white
🐖🐓🐄bony & aggressive
🍌seeds!
But that’s not how evolution usually works. Most of the time, the main thing that natural selection does is keep things the same. If you think about how old the Earth is, the really surprising pattern is how little change has occurred. Let’s think back to the days of the giant dinosaurs.🦕🦖🦕🦖🦕🦖🦕🦖🦕🦖🦕🦖
100 million years ago, our ancestor was a small fuzzy creature that also gave rise to lots of other mammals.🐭→🦇🦌🐬🦏🐆🐒🐩🐘👦🏽
One of the many differences between it and us is that our bodies are larger. So over time, body size increased in the lineage leading to us.🐭 → 🐭
Small fuzzy creatures beget large fuzzy creatures
How fast did it evolve? Imagine that it became 1% larger every 10,000 years. That doesn't sound like a particularly fast rate of evolution, and certainly we've measured faster rates in other species. If the animal started out weighing 1 pound, then after 100 million years it would weigh 1043 pounds. About as much as the Milky Way Galaxy:
⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨🌟☀️⭐✨⭐✨🌟☀️⭐✨🌟☀️
🌌🐭
A very large fuzzy creature
Clearly that didn’t happen. Instead, it evolved much more slowly. That’s because the most common thing for natural selection to do is to constrain change, not promote it. That’s not really surprising, because organisms are already really well optimized to Earth’s environment. The vast majority of mutations will just screw things up and reduce the fitness of the organism. So, natural selection will weed the mutations out.
🍎🍎🍎🍎→🍎🍏🍎🍎→🍎🍎🍎🍎
The fate of most mutations
This means that from first principles, you shouldn’t expect human genomes to have changed very much since our species first appeared. So what’s a better way to envision human populations? Humans first evolved in Africa. The highest levels of human genetic diversity are still seen in Africa. Non-Africans represent a subset of this diversity. Both Africans and non-Africans have evolved a little bit since humans started to spread across the planet. Most genetic variants, though, are similarly abundant among all continents. All humans are unique combinations of this shared gene pool, which Chapter 2 will examine.