How climate change made humans who we are

I think we are in trouble I think one of our greatest tricks in the deep past was moving on," he said. "We don't really have that trick anymore because the planet is full."

"What we do have that I think is probably unprecedented," he said, "is the ability to consider the future and modify our behaviour in the present."

The Badjoa of the Philippines, the last remaining free-diving nomads in the world, who can spend several minutes at a time holding their breath 100 metres underwater as they gather food from the sea floor.

We have not eliminated genetic variation among humans. If anything, the human genome shows that variation has gone up with recent population explosion throwing up ever more mutations within the last 200 generations. We may therefore be evolving faster than before. Among them are mutations which keep the production of lactase (the enzyme for digesting milk) switched on throughout life. This persistence of lactase, beneficial after we domesticated cattle, first arose about 6000 years ago in Europe. It later evolved independently, through mutations in different genes in separate pastoral populations in Africa.

Colonising new environments favours faster reproduction apparently causing women to give birth at younger ages. Easy availability of calories also makes puberty set in earlier in richer countries. Meanwhile both natural and sexual selection remain quite active even in European populations through the industrial era.

Infectious diseases continue to exert selective pressures on our genes, in an evolutionary arms race that is challenging modern medicine as pathogens become resistant to drugs. Some regions of our genomes, such as that corresponding to the immune systems, are more variable and evolve more rapidly than other regions. For instance, the haemoglobin mutation causing sickle cell anaemia also confers resistance against malaria and is therefore favoured in regions where the latter is prevalent. Newer techniques allow us to find genetic variants in proteins that confer some resistance against other diseases (for example lassa fever and HIV) clearly resulting from recent selection.

Microbiology raises new questions about what it even means to be physically human, because our bodies teem with far more bacteria than human cells. This microbiome plays a critical role in our health. Many of these bacteria are subject to selection from changing human cultures and environments. A gene producing an enzyme that helps marine bacteria break down the cell wall of seaweed algae was recently found in the gut bacteria of Japanese sushi-eaters – a remarkable example of gene exchange among bacteria conferring a selective advantage to humans.

"Thanos" said


aka Obed Marsh's grandkids from Innsmouth.

Lots of readers have asked me to react to this Telegraph article: “Sir David Attenborough: Humans have stopped evolving”:

"We stopped natural selection as soon as we started being able to rear 9599 per cent of our babies that are born.
We are the only species to have put a halt to natural selection, of its own free will, as it were."
Well, obviously he’s wrong.

First, should we discount the rapid evolution of the last 10,000 years? My post from several years ago describes this finding (“How human evolution accelerated”).

Admittedly, many of the ecological factors that were important in that time frame are no longer selecting in the same way on human populations. It’s not obvious, for example, that milk drinking is correlated with higher offspring number in today’s industrialized societies, or that the speed of starch breakdown in saliva has anything to do with survival today.

But malaria does. It remains the top pathogen for human misery in the world, killing millions every year and debilitating many, many more. More than two dozen genes in human populations have common alleles that help to protect against malaria, from the well known (sickle-cell) to the obscure (FLT1). Natural selection is mercilessly maintaining them in human populations. For some of these genes, natural selection is maintaining an equilibrium, with deaths from malaria balancing deaths in homozygotes who carry a harmful genotype. Even populations where malaria has been eradicated in the last hundred years, like the United States, still suffer natural selection on some of these genes. The sickle cell allele has been declining in frequency among African Americans for the last hundred years, as a result of selection against the sickle allele in homozygotes.

People who talk about “natural selection stopping” in humans are usually forgetting about most of the world.

They’re also forgetting about the other side of selection’s two-faced coin: fertility. Not having children, or not bringing a pregnancy to term, allow selection just as death does. Fertility differences have become marked in today’s industrialized societies, as people delay starting families into their 30s or 40s, or decide not to have children at all.

The Framingham study, in the town of Framingham, Massachusetts, is one of several that have examined phenotypes in large samples of people for long enough to examine fitness outcomes. Measure somebody when young, wait a long time, and see how many children they had. This is a measure of fitness, one that does not ignore the fertility component.

As studied by Stephen Stearns and colleagues, such studies have shown consistent and strong evidence for natural selection in the populations of the last fifty years. From a 2010 review paper Stearns:2010:

These studies report three striking findings. First, both women and men are under selection for earlier age at first birth in both pre-industrial, natural fertility populations and in post-industrial populations. Second, women are under selection for later age at last birth in a pre-industrial population and later age at menopause in two post-industrial populations. Third, women are under selection for increased height in one pre-industrial population and for decreased height in three post-industrial populations.
And about saliva, above, I wrote “not obvious” because correlations between fitness and diet-related genes today in human populations are not inconceivable. For example, the Framingham study showed that weight in American women has been positively selected over the last 50 years, while blood glucose level has been under stabilizing selection – extreme high and low blood sugar levels having fewer offspring. No one has yet investigated whether salivary amylase or lactase affect either phenotype in Americans today. Both phenotypes are heritable in this population, meaning that some additive variation in genes underlies the total variation. Maybe someday we’ll learn which genes are involved.

The next phase of human genetics will see the sequencing of hundreds of thousands of genomes. The UK has already embarked upon a project to acquire more than 100,000 genomes, and we can anticipate the addition of genetic sampling to vast long-term health studies in other nations. These studies are intended to discover associations between rare alleles and disease traits. Their huge sample size means that they will be much more powerful in identifying correlations between alleles and fitness than we have for any population in nature. We will begin to see an unparalleled picture of evolution in action, in human societies, now.

What will this do for our concept of humanity?

I expect it will be an enormous opportunity for biology education, and for anthropology. The traits that make a difference to selection are not typically the ones that matter to the health of 70-year-olds. What we prioritize, the way we shape our lives, affects the evolutionary future of our species – maybe in surprising ways.

"ShepherdsDog" said


aka Obed Marsh's grandkids from Innsmouth.

"Zipperfish" said
I've been around the world and seen that only stupid people are breeding.