The clues to human uniqueness
Over 2 million years ago, before the emergence of the genus Homo, within the rift valleys and savannah grasslands of Africa during the Pliocene period, a unique event took place. One that, with some hyperbole, admittedly, shaped the course of human evolution. The event was on a molecular scale but had its bearings on what we now call and search for as the “human condition”.
Ever since Darwinian evolution gave us an image of our nearest cousins (the chimpanzees, bonobos, gorillas and orangutans of the non-human hominids) we have been constantly searching for what it means to be human — that human condition — for what makes us unique. From an opposable thumb, to the way our brains work, to the simple fact we learnt to cook our food. All of these have been proposed — at one point or another and with various degrees of certitude — as the key to our humanity.
Now the indication lies not so much within us, but with our interaction with disease. In a paper published in the Proceedings of the National Academy of Sciences, researchers are circling in on another hypothesis. The suggestion that inactivation of two specific genes related to the immune system may have conferred selected ancestors of modern humans with improved protection from certain infectious diseases, and also lead to our divergence from our common ancestors.
Dr Ajit Varki and his team study the biology of sialic acids. What they found was that the inactivation of two sialic acid genes played a role in the course of human evolution. Two events at the genomic level that occurred before the emergence of modern humans in Africa.
Infectious pathogens interact with the different types of sialic acids on your cell surface. They are the target for a huge number of pathogens, and successful pathogens have evolved to be able to mimic the sialic acid coat to evade and dampen the human immune system. Look at this in an evolutionary context and what you get is an evolutionary arms race of sorts, with repeated re-invention of the microbes and pathogens that interacted with our ancestors.
Dr Varki and his team describe two sialic acids (Neu5Ac and Neu5Gc) — molecules that differ by a single oxygen atom — one of which was rendered nonfunctional by genetic events during hominin evolution. Across the range of non-human hominids both Neu5Ac and Neu5Gc exist. Uniquely, modern humans do not have Neu5Gc. Today we see the traces of this evolutionary event when we look at the disease-specific differences between humans and the great apes.
There are a large number of disease difference between humans and primates that are not explained by anatomical differences. HIV progression to AIDS is common in humans but very rare in apes, myocardial infarction is common in humans but rare in apes, and humans are susceptible to the malaria parasite P. falciparum whereas apes are resistant.
Falciparum is a recent disease as all the falciparum strains of the world come in one small clade in Africa, evolving from an older strain that had a preference to bind and infect via Neu5Gc. When early hominids evolved to get rid of Neu5Gc and have mainly Neu5Ac (possibly as a result of selective pressure from disease), and perhaps resulting in a potentially malaria-free period in time, until that is falciparum evolved to keep up.
The difference between Neu5Gc and Neu5Ac point to powerful evidence of how infectious diseases have shaped human origin. Researchers make the argument that changes of innate immune defense against invasive human pathogens, particularly those that are involved in neonatal life, would have exerted a powerful selection pressure on the reproductive success of our ancestors.
In a press release Varki noted that it’s probable that humanity’s evolutionary bottleneck was the complex result of multiple, interacting factors. “Speciation is driven by many things,” he said. “We think infectious agents are one of them.”
Image — source
1 thought on “The clues to human uniqueness”
Thanks a lot for the information, but please don’t start with a mistake: “… before the emergence of the genus Homo, within the rift valleys and savannah grasslands of Africa during the Pliocene period, a unique event took place …” It is now commonly accepted among paleo-anthroplogists that our evolution did not take place on the savannah grasslands.
In 1995, professor Phillip Tobias, in his Daryll Forde Memorial Lecture at University College, London, stated of the Savannah Hypothesis: “We were all profoundly and unutterably wrong! All the former savannah supporters including myself must now swallow our earlier words …” In spite of this, the savannah ideas are apparently still taken for granted in most popular books and articles on human evolution. The decease of professor Tobias must be an opportunity for paleo-anthropologists to finally get rid of the dry savannah fantasies. If some hominids lived in savannahs, it must have been along the rivers, swamps and lakes there.
It was Tobias’ predecessor at the Witwatersrand University, Raymond Dart, who helped the savannah ideas to become generally accepted. In the 1920s, geologists thought the climate in South Africa had not changed since the Pliocene, so Dart concluded that the Taung child (a human ancestor, he believed) had lived in dry grasslands. We now know that Taung was possibly no human ancestor, and moreover lived in “a more forested habitat, with denser cover along waterways” (Berger & Clarke 1995). For an updated view on ape and australopith evolution, google “aquarboreal”.
Homo erectus, with its extremely heavy skeleton (i.e. pachyostosis, typical of littoral tetrapods), could not have practiced “endurance running” (a still popular idea, even among paleo-anthropologists). Apparently, archaic Homo spread to different continents and even islands such as Flores along coasts (Littoral Hypothesis), and from there inland along rivers. At and in the water, these dextrous omnivores butchered drowned ungulates and stranded whales, and collected shell- and crayfish, and other waterside foods rich in brain-specific nutrients such as DHA (Cunnane 2005). For an updated view on Homo evolution, google “econiche Homo”. For a short slide show on the littoral theory, google “vaneechoutte pediatrics”.
One of the last publications of Professor Tobias was “Revisiting Water and Hominin Evolution”, the first chapter of an ebook devoted to our ancestors’ waterside evolution:
Mario Vaneechoutte, Algis Kuliukas & Marc Verhaegen eds 2011 Bentham Science Publications,
“Was Man More Aquatic in the Past? Fifty Years after Alister Hardy: Waterside Hypotheses of Human Evolution”.
Although it is clear that Pleistocene Homo was much more aquatic than Homo sapiens is today, how aquatic they were is still debatable. Any scientific discussion of human evolution should take into account this ebook, which contains contributions of all major proponents of waterside hypotheses.
Comments are closed.