The Pleistocene transition to meat-eating increased spillover of zoonotic diseases

Ed Hagen

Washington State University

February 26, 2025

My research: Evolved psychology of plant drug use

Food and Agriculture Organization (2019)

My hypothesis

Intensifying zoonotic pathogen pressure in the Pleistocene selected for increased pharmacological plant use

Rabies as a model zoonotic disease

  • Seven lyssaviruses infect humans
  • Transmission: bites from infected bats and carnivores
  • Infects the nervous system
  • 100% lethal (without immediate treatment)
  • 13K–59K annual deaths globally (mainly from rabid dogs)
  • No human-to-human transmission

Evolution of the human diet

Hominin Plio-Pleistocene ecology

Miocene diet

Hominin Plio-Pleistocene ecology

Pliocene diet

Pliocene diet

Hominin Plio-Pleistocene ecology

The transition to carnivory

The transition to carnivory

Domı́nguez-Rodrigo et al. (2021)

Dramatic dietary shift

Plant vs. animal pathogens

Kim et al. (2020)

Zoonotic virus entry & replication

Palakurty and Diamond (2025)

Most animal RNA viruses enter human cells

Dufloo et al. (2025)

Wild mammalian zoonotic reservoirs

Han, Kramer, and Drake (2016)

Bushmeat spillover risk

  • Ebola, HIV, and monkeypox, SARS-CoV-1, and possibly SARS-CoV-2 (Kurpiers et al., 2016; Peros et al., 2021).

  • Systematic review: 133 reports of disease involving 60 pathogens in 58 bushmeat species, mostly mammals (95%), with some reptiles (4%) and birds (1%).

  • The most common zoonotic pathogens were helminths (37%) and bacteria (33%), followed by viruses and protozoa (15% each) (Peros et al., 2021).

Hunting spillover risk

  • Congo Basin foragers (pygmies) are at higher risk of zoonotic spillover than neighboring farmers
  • Simian foamy virus (SFV)
  • Human T-lymphotropic virus type 1 (HTLV-1)
  • Monkeypox
  • Ebola (highest prevalence of antibodies ever reported: 18.7% vs. 2-3.5%)
  • Severe bites from a non-human primate are major risk factor for infection

RNA virus tissue tropism and transmission

Brierley, Pedersen, and Woolhouse (2019)

RNA virus virulence

Leaves: correct classifications.

Brierley, Pedersen, and Woolhouse (2019)

Zoonotic virus case fatality rates

Guth et al. (2019)

Concluding remarks

  • The human niche involved fewer anti-infective plant foods and more infective animal foods
  • Theories often emphasize meat spoilage but ignore spillover risk
  • Zoonotic pathogens often have poor transmissibility but high virulence
  • A single lifetime infection of a rabies-like pathogen would have had an enormous impact on fitness
  • Theories of behavioral immunity need to consider less risk from sick people and more risk from bites, cuts, insect vectors, and raw meat.

References

Brierley, Liam, Amy B. Pedersen, and Mark E. J. Woolhouse. 2019. “Tissue Tropism and Transmission Ecology Predict Virulence of Human RNA Viruses.” PLOS Biology 17 (11): e3000206. https://doi.org/10.1371/journal.pbio.3000206.
Domı́nguez-Rodrigo, Manuel, Enrique Baquedano, Elia Organista, Lucı́a Cobo-Sánchez, Audax Mabulla, Vivek Maskara, Agness Gidna, et al. 2021. “Early Pleistocene Faunivorous Hominins Were Not Kleptoparasitic, and This Impacted the Evolution of Human Anatomy and Socio-Ecology.” Scientific Reports 11 (1): 16135.
Dufloo, Jérémy, Iván Andreu-Moreno, Jorge Moreno-Garcı́a, Ana Valero-Rello, and Rafael Sanjuán. 2025. “Receptor-Binding Proteins from Animal Viruses Are Broadly Compatible with Human Cell Entry Factors.” Nature Microbiology, 1–15.
Food and Agriculture Organization. 2019. “FAOSTAT.” http://www.fao.org/faostat/en/#home.
Guth, Sarah, Elisa Visher, Mike Boots, and Cara E. Brook. 2019. “Host Phylogenetic Distance Drives Trends in Virus Virulence and Transmissibility Across the Animal-Human Interface.” Philosophical Transactions of the Royal Society B: Biological Sciences 374 (1782): 20190296. https://doi.org/10.1098/rstb.2019.0296.
Han, Barbara A, Andrew M Kramer, and John M Drake. 2016. “Global Patterns of Zoonotic Disease in Mammals.” Trends in Parasitology 32 (7): 565–77.
Kim, Jun-Seob, Sung-Jin Yoon, Young-Jun Park, Seon-Yeong Kim, and Choong-Min Ryu. 2020. “Crossing the Kingdom Border: Human Diseases Caused by Plant Pathogens.” Environmental Microbiology 22 (7): 2485–95.
Palakurty, Sathvik, and Michael S Diamond. 2025. “Viral Entry as a Low Barrier to Zoonosis.” Nature Microbiology, 1–2.