Saturday, June 29, 2019


Did Humans Originate From Manipulation By Parasites?
Post #29
Donald A. Windsor

Humans are very different from other hominid primates. How did we get this way?

The Savanna Hypothesis postulates that our ancient ancestors abandoned their arboreal life to live in a savanna, a habitat with mostly grass and few trees. It attributes our upright posture and bipedalism to this change of venue. Our keen intellect, organizational skills, and language evolved in this savanna habitat (1).

I suspect that parasites instigated this transition from arboreal to terrestrial.

My suspicions were aroused by the model of the pill bug and the bird. The acanthocephalan Plagiorhynchus cylindraceus is parasitic in its definitive host, the European Starling Sturnus vulgaris, and in its intermediate host, the isopod "pill bug" Armadillidiuum vulgare. The parasite manipulates the pill bug to behave in ways that increase its chances of being eaten by the bird, Normally, pill bugs prefer dark, humid, covered places. Infected pill bugs tend to wander out into the open where it is lighter and drier, where they become easy prey for the bird (2).

Could some arboreal hominids have been manipulated by a parasite to venture into the more dangerous savanna? How similar is the pill bug and bird example to an early hominid coming down from the safety of an arboreal lifestyle to roam in the dangerously open savanna? While there may be no way of knowing for certain, the possibility is intriguing.

Consider that the parasite Toxoplasma gondii is notorious for manipulating humans. Moreover, its definitive host is felines (3). Large feline predators inhabit grasslands. Its intermediate host is an array of prey animals, especially rodents (4). Humans could be an intermediate host.

I pose this hypothetical parasite involvement because the Savanna Hypothesis has been discussed for over two centuries and, while it may be out of favor now, it still has merits. Besides, numerous migrations allowed humans to become a cosmopolitan species. Parasites could well have been instrumental in moving people around.

References cited:

1. Anon. Savannah hypothesis. Wikipedia 2019 June 9: 1-7.
2. Moore, Janice. Responses of an avian predator and its isopod prey to an acanthocephalan parasite. Ecology 1983 October; 64(5): 1000-1015.
3. Anon. Toxoplasma gondii. Wikipedia 2019 June 13: 1-23.
4. Tenter, Astrid M. ; Heckeroth, Anja R. ; Weiss, Louis M. Toxoplasma gondii: from animals to humans. International Journal for Parasitology 2000 November: 30(12-13): 1217-1258.

Wednesday, June 12, 2019


Hemiparasites Benefit Their Hosts At The Species And Ecosystem Levels
Post #28
Donald A. Windsor

Hemiparasites are photosynthetic plants that parasitize free-living plants to obtain nutrients (1).

Growth of the host plants can be stunted by the hemiparasites, thus allowing more sunlight to reach lower growing plants and increasing biodiversity.

Hemiparasites benefit prairie ecosystems similar to the way grazing animals do, by preventing the taller plants from taking over. For this reason they are sometimes called "pseudograzers" (2).

Individual hemiparasites harm their individual hosts, but the hemiparasites as species benefit other species, and the resulting biodiversity benefits the ecosystem. All species in the ecosystem benefit from the ecosystem staying intact.

References cited:

1. Těšitel, Jakub ; Plavcova, Lenka ; Cameron, Duncan. Interactions between hemiparasitic plants and their hosts: The importance of organic carbon transfer. Plant Signaling & Behavior 2010 August; 5(9): 1072-1076.

2. DiGiovanni, Jane P. ; Wysocki, William P. ; Burke, Sean V. ; Duvall, Melvin R. ; Barber, Nicholas A. The role of hemiparasitic plants: influencing tallgrass prairie quality, diversity, and structure. Restoration Ecology 2017 May; 25(3): 405-413.