It is thought that much human evolution occurred during the Pleistocene – 2.8 million to 12 thousand years ago. What are the consequences of genes optimized for a Pleistocene environment now expressed in the modern environment? We have new technology, petro-chemicals, artificial light, recreational drugs. In particular, we consume radically different food than our predecessors. Are we healthier if we eat like we did in the Pleistocene?

Pleistocene scene

This thinking has led to a Paleo lifestyle movement that advocates for a Paleo diet. This concept has become popular in part because it prioritizes eating meat and some high fat foods. But is this really the healthiest option, and the best evolutionary insight as to what we should be eating? What exactly is the Paleo diet, and how much should we worry about eating only foods that we were evolved to eat?

The Paleo diet raises additional questions. There is tremendous diversity in modern human diets, even among traditional human populations.  Which stone age populations should we emulate? Can we really know how our Pleistocene ancestors ate with any certainty? How far back should we go – 1,000 years, 10,000 years, 100,000 years -to pinpoint the healthiest evolved diet for humans? Can modern hunter-gatherers, like the Hadza of Tanzania be a stand-in for our ancestral hominins in terms of diet?

Hadza consuming honeycomb

[Students – check back this weekend for journal club assignments. You will be expected to present your journal club critiques on October 4th to give you sufficient time. We will be discussing Paleo diets on September 27th]

1)  Eaton (2006)-Ancestral human diet

2) Paleofantasy by Marlene Zuk

3) Genne-Bacon Thinking evolutionarily about obesity.

4) Paleo microbiota

Assignment due September 27th: Of the different sources of animal protein, fish seems to be very healthy, maybe even the healthiest, in the human diet. Elaine Morgan, an Oxford trained anthropologist, argues that early humans had an aquatic phase – the so called “aquatic ape” hypothesis, first proposed by marine biologist Alister Hardy. Morgan, Hardy, and others propose that early aquatic humans spent a lot of time in the water, explaining our lack of hair and tails, the ability to hold our breath, our upright posture good for wading, and our air filled sinuses. Aquatic apes would have eaten a lot of seafood, rich in omega-3 fatty acids, useful in brain development, maybe even permitting humans to evolve large brains. Argue for or against the idea that an aquatic phase for humanity explains why fish, and marine omega-3 fatty acids, are healthy for us humans.

Useful links:

“We got smart from eating fish and living in water”- Japan Times

Aquatic explanation for the sinuses

Wired magazine Sorry David Attenborough

Aquarboreal ancestors

John Hawks on the Aquatic Ape Hypothesis

Are we masters of our own fate… or have our brains been zombified by microbes?

Cordyceps fungus takes over the brains of ant hosts. Infected ants that usually inhabit the ground are manipulated by the fungus – they climb tall vegetation where a fruiting body of the fungus emerges from their heads.

To learn more about Cordyceps and similar mind-controlling parasites, watch Zombie parasites on YouTube.

Dethlefson and colleagues wrote that “from the microbial perspective…the distinction between human health and disease is important only as far as it affects microbial fitness.

This raises the possibility that microbes manipulate us for their own benefit. Examples abound of parasites like Cordyceps exploiting their insect hosts. But do microbial parasites affect us humans in the same way?

Dethlefson and colleagues write that microbiota are “prevented (usually) from exploiting the host to obtain additional resources,” citing the immune system as “the most conspicuous set of anti-exploitation adaptations involved in human–microbial symbiosis.”

The red queen effect

The fact that we need immune “anti-exploitation” adaptations is a clue that we are engaged in a never-ending arms with harmful microorganisms.

But the immune system only gets us so far. For example, infection by the protozoan Toxoplasmosis provides an excellent example of a parasite that manipulates the brain and behavior of mammals that it infects. Infected rabbits for instance lose their innate fear of bobcat urine. Rabbits that are attracted to feline predators have a distinct disadvantage when it comes to fitness. Not so for the parasite. It’s fitness is increased as it completes its life cycle. Our closest relative, the chimpanzee, also seeks out the urine of their main predators, leopards, when infected by Toxo. In the United States, almost a quarter of human adults are infected with Toxoplasmosis. Problem? Perhaps.

Leopard pee – attractive to infected Toxo-infected Chimps

How about our microbiomes, the complex microbial communities that mainly inhabit our guts? Are they exploiting our brains and behaviors? Indeed, microbes are suspected to be involved in depression and anxiety, demonstrated experimentally in mice. Is this manipulation?

Marble burying anxious mice

Bruce-Keller (2015) and colleagues showed that the microbiome is responsible for marble burying behavior in mice that were fed a high fat diet (HFD). Anxious mice like to bury marbles. The implication of this study is that microbes can make mice anxious and depressed.

Does this mean that microbes cause depression in people? Drawing on these kinds of studies, investigators have given probiotic microbes to people with depression. They seemed to get better!

Depression score was lower in patients treated with Lactobacillus.

Akkesheh and colleagues gave  Lactobacillus acidophilus and Lactobacillus casei and Bifdobacteria to depressed patients in Iran. Probiotics apparently made patients less depressed. (The dark blue bar indicates placebo, and the light blue bar indicates the probiotic group. They used the Beckwith Depression Inventory to measure depression.)

How might toxoplasmosis affect chimp brains? How does Lactobacillus brighten mood? We don’t know, but there are clues.

Bacteria make neurotransmitters – the same ones that we use:

Lyte PharmaNutrition 2013

Presumably bacteria evolved the signaling molecules like GABA, norepinephrine, and serotonin first. We eukaryotes might have evolved nervous systems using the same molecules because they conveyed useful information about the doings of microbes (but who knows.)

My research collaborators proposed that gut microbes might affect what we decide to eat.

1) How Pernicious Parasites Turn Victims into Zombies.

“Joanne Webster and colleagues from Imperial College, UK, and the University of Leeds, UK, make the case that Toxoplasma may be a contributory factor in some cases of human schizophrenia given its presence in the brains of infected individuals and our long lifespan”

2) Alcock J, Maley C, Aktipis A. (2014) Is eating behavior manipulated by the gastrointestinal microbiota? Evolutionary pressures and potential mechanisms. Bioessays.

” Like microscopic puppetmasters, microbes may control the eating behavior of hosts through a number of potential mechanisms, including reward pathways, production of toxins that affect mood, and hijacking of neurotransmitters via the vagus nerve”

3)  Melancholic Microbes: a link between gut microbiota and depression?

“Whether fecal transplantation of microbiota from animals or other models of depression could also alter behavior of the recipient is an intriguing possibility”

4) Sutterland_Toxoplasmosis gondii in schizophrenia, bipolar disorder and addiction

“A significant OR of 1.91 (95% CI 1.49–2.44, P < 0.00001) was found, indicating an increased prevalence of T. gondii infection in heroin addiction”

Writing for next Tuesday, September 13th (pick one):