Humans were Domesticated by C4 photosynthetic CO2 sequestration GRASS

Rowan Sage

Lamont-Doherty Earth Observatory

Adjunct Senior Research Scientist

Lamont-Doherty Earth Observatory

Biology and Paleo Environment

University of Toronto

Dept of Ecology and Evolutionary Biology

Mail Code: 25 Willcocks Street

Toronto ON M5S3B2

Canada

Phone: 

(416) 978- 7660

 rfs2140@columbia.edu

 The spread of the savanna grasses may have been a precondition for the evolution of the traits that characterize our genus, Homo, suggested Rowan Sage, of the University of Toronto, at a conference on human evolution and climate change at Lamont-Doherty Earth Observatory here in New York on Thursday (April 19).

 https://www.livescience.com/19823-grass-fed-evolution-altered-environment-fueled-human-change.html

After about 35 million years ago, an upgraded version of photosynthesis emerged, called the C4 pathway. And in time the plants that employed it, including grasses and sedges, spread. A declining level of carbon dioxide in the atmosphere appears to have been responsible for the switch to C4. Chemical evidence suggests the greenhouse gas declined over tens of millions of years.

C4 plants could use carbon dioxide more efficiently to feed themselves, and they used less water, making them well adapted to dry environments, like those favored during the Pliocene Epoch, which began about 5.3 million years ago, when grasslands and savannas spread.

As a potential source of food, the C4 plants weren't the best innovation. They offered tough, fibrous leaves, small-seeded grains and small, fibrous rhizomes. Even today, only a few dozen C4 plants, most significantly maize and sugar cane, are cultivated for food.

But some other animals were better adapted to digesting these plants. Ruminants, such as cows and buffalo, regurgitate and re-chew a tough meal, Sage said.

"So the possibility is human evolution was driven by our ancestors going out and taking down the (animals) who could digest the C4 vegetation. But they would have to deal with these guys," he said, referring to a photo of a lion during his presentation.

So, given the option of eating the C4-eaters and the need to avoid predators, early humans may have evolved an upright stature, begun running long distances, carrying out extended hunts in groups and defending against predators, and evolved other distinctly human traits, he said.

 https://www.researchgate.net/publication/320519687_Photosynthetic_pathway_of_grass_fossils_from_the_upper_Miocene_Dove_Spring_Formation_Mojave_Desert_California

 The spread of grasslands in the Miocene and of C4 grasses in the late Miocene-Pliocene represents a major development in terrestrial plant evolution that affected the climate system and faunal evolution

The family of grasses (Poaceae) is highly diverse with up to 11,000

species and largely dominates open habitat ecosystems around the

world (Gibson, 2009). Within grasses, a specialized photosynthetic

pathway for fixating CO2 from the atmosphere evolved relatively re-

cently. This pathway, known as C4 photosynthesis, is an adaptation to

the ancestral C3 photosynthetic pathway that acts to mitigate the effects

of photorespiration that can occur in response to ecological pressures in

C3 photosynthesis. C4 has evolved at least 62 different times, resulting

in one of the best examples of convergent evolution (Sage et al., 2011,

2012).

  C4 shade-tolerant species exhibit similar performance as shade-tolerant C3 species in terms of quantum yield, steady-state photosynthetic

 https://www.researchgate.net/publication/320519687_Photosynthetic_pathway_of_grass_fossils_from_the_upper_Miocene_Dove_Spring_Formation_Mojave_Desert_California

C4 grasses are both anatomically and biochemically distinct from C3 grasses. In C3 plants, both carbon assimilation and carbon reduction occur in the mesophyll cells. In C4 plants, these processes are spatially separated resulting in the internal concentration of CO2, where CO2 is assimilated into mesophyll cells and then shuttled into vascular bundle sheath cells where carbon reduction occurs via the Calvin cycle (Hatch,1987). 

 The biochemical compartmentalization of C4 photosynthesis commonly results in a wreath-like structure, when viewed in cross section, known as “Kranz”anatomy in which mesophyll cells wrap

around an inner layer of bundle sheath cells (Brown, 1975; Hatch, 1987). Almost all known C4 plant lineages have Kranz anatomy (Bowes,2011; Sage et al., 2011; Voznesenskaya et al., 2001). The ability of the C4 pathway to concentrate CO2 around the photosynthetic enzyme, Rubisco, confers a competitive advantage over the C3 photosynthetic pathway in warm, dry and/or low CO2 conditions (Ehleringer et al.,1997). As a result of this process, the organic carbon isotope value of C4 plants is more 13 C-enriched than co-occurring C3 plants. Therefore both visual as well as isotopic evidence can be used to distinguish between the C3 and C4 photosynthetic pathway.

Quo vadis C4? An ecophysiological perspective on global change and the future of C4 plants

C₄ plants are directly affected by all major global change parameters, often in a manner that is distinct from that of C₃ plants. Rising CO₂ generally stimulates C₃ photosynthesis more than C₄, but C₄ species still exhibit positive responses, particularly at elevated temperature and arid conditions where they are currently common. Acclimation of photosynthesis to high CO₂ occurs in both C₃ and C₄ plants, most notably in nutrient-limited situations. High CO₂ aggravates nitrogen limitations and in doing so may favor C₄ species, which have greater photosynthetic nitrogen use efficiency. C₄ photosynthesis is favored by high temperature, but global warming will not necessarily favor C₄ over C₃ plants because the timing of warming could be more critical than the warming itself. C₃ species will likely be favored where harsh winter climates are moderated, particularly where hot summers also become drier and less favorable to C₄ plant growth. Eutrophication of soils by nitrogen deposition generally favors C₃ species by offsetting the superior nitrogen use efficiency of C₄ species; this should allow C₃ species to expand at the expense of C₄ plants. Land-use change and biotic invasions are also important global change factors that affect the future of C₄ plants. Human exploitation of forested landscapes favors C₄ species at low latitude by removing woody competitors and opening gaps in which C₄ grasses can establish. Invasive C₄ grasses are causing widespread forest loss in Asia, the Americas and Oceania by accelerating fire cycles and reducing soil nutrient status. Once established, weedy C₄ grasses can prevent woodland establishment, and thus arrest ecological succession. In sum, in the future, certain C₄ plants will prosper at the expense of C₃ species, and should be able to adjust to the changes the future brings.

  https://www.cnbc.com/2019/06/11/this-is-a-15-trillion-opportunity-for-farmers-to-fight-climate-change.html

 “The potential for agricultural soils to capture and store atmospheric carbon dioxide is the most hopeful solution I know of to address climate change,” said David Perry, Indigo’s CEO. “The technology and know-how for regenerative farming already exists, so we can begin to make a difference right now.”