Tag Archives: grass

Grassroots and fisticuffs

A couple of recent perspectives on Australopithecus and early Homo:

Tropical grasses and sedges commonly use the C4 pathway for photosynthesis, which evolved from the C3 pathway used by most plants, in response to lower atmospheric carbon dioxide levels. We can trace an isotopic signature in the teeth of animals that eat C4 plants (and animals that eat those animals). From about 3.5 Mya we see evidence of hominins eating more C4 foods: either grasses (maybe including underground parts) or animals that ate grasses. Paranthropus boisei seems to be a particular champion with the C4s. Chimpanzees, even when they live in open woodlands, mostly stick to eating fruits and leaves. By contrast, exploiting grassroots and sedges to varying degrees may be an important component of australopithecine adaptation to the savannah.

Not only do australopithecines have enlarged back teeth relative to great apes, but they also have smaller canines. Canines are mostly good for fighting, so it’s nice to think that canine reduction is a sign of our ancestors becoming more peaceful. Could be. But it’s possible they just switched fighting styles. David Carrier argues that australopithecine faces are built to withstand punches, starting from the time that australopithecine hands were capable of forming fists. This might go together with high levels of sexual dimorphism – males, the more violent sex, being bigger than females (but early hominin sexual dimorphism is a contentious subject.)

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Grasses and gases for a cooling world

The standard sort of photosynthesis uses a so-called C-3 chemical pathway. But  maybe from 8-7 million years ago there’s an increasing proliferation of so-called C-4 plants. They use an alternative, more-efficient pathway to incorporate carbon from C02. C-4 plants evolved independently 45-60 times.

Tropical grasslands are mostly C-4. The profusion of grasses, herbivores, and carnivores on tropical savannahs will owe a lot to C-4 plants. This evolutionary transition is probably a sign that C02 levels are declining, and have reached a threshold where C-4 plants are favored.

Going back about 7 million years, C02 levels stood at maybe 1500 parts per million (ppm). (They were higher earlier in the Miocene.) Levels decline pretty steadily, leading to global cooling and eventual Ice Ages. But things never again reach the extremes of Snowball Earth 750 million years ago.

At the beginning of the Industrial Age C02 levels stood below 300 ppm. They went above 400 ppm last year.

Land of the throats

17.3-16.3 million years ago

There’s a great expansion in the diversity of horses in the mid-Miocene, especially horses that are adapted to grazing rather than browsing. The shift to grazing is going on world wide among many different groups. In South America the big grazers are the liptoterns, ungulates not closely related to horses that evolve to look a lot like them, with high-crowned grazing teeth, single-toed hoofed feet and legs built for speed. (Edgar Rice Burroughs took the name thoat – what his characters rode around on on Barsoom/Mars — from one genus of liptotern, Thoatherium.)

thoat

We often think of evolution as a matter of organisms adapting to their environments, but when the environment is other organisms, each side may be chasing a moving target. Or sometimes the sides may reach an equilibrium. In the case of grazing animals, there’s a process of coevolution that goes on between grazers and grasses. Where grazers are active, the plants that survive are grasses, which keep leaves above the ground but grow from underground. And this works in the other direction: in moderately dry climates, grasses are more productive than taller brushy plants, so it’s when grasses take over that there’s enough food around for grazers – a mutually reinforcing cycle. With drier climates from the mid-Miocene on, grasslands and grazers get to be more and more important.

So a lot of the story of life on Earth is not just the appearance of this or that cool animal, but also the evolution of ecosystems. At the same time grasslands were spreading on land, for example, kelp forests were spreading in coastal oceans. We’ll see how important grasslands are in human evolution and history. And kelp forests, with their rich fish populations, might have been important too, as the highway that the earliest Americans followed along the Pacific coast to the New World.

Leaves of grass

Not as dramatic as the evolution of Triceratops, Tarchea giganta, or T. rex, but of more lasting consequence, is the evolution of grasses (Poaceae). We know from coprolites – fossil feces — that grass was around by the Late Cretaceous, so the coevolution of grass and grazers had already begun with dinosaurs. These early grasses were not widespread. It would take climate shifts and more evolution (toward using carbon dioxide more efficiently) to create the sort of grasslands we are familiar with.

Grasses have played a central role in human evolution and human history. Human beings evolved in tropical grasslands, and some evolutionary psychologists think we still have an instinctive affinity for this environment. The domestication of grasses (wheat, barley, oats, millet, rice, corn) was one of the great revolutions in human prehistory, and grasses provided most of the calories people ate for most of recorded history. Contact along the frontier between grasslands supporting pastoralists and grain growing lands supporting peasants is one of the great engines of historical dynamics.

Grasses grow from the base of the leaf, not the tip of the stem, which is what allows them to recover from being grazed. This makes them a recurring symbol both of the transitoriness of life (“All flesh is grass, and all the goodliness thereof is like the flower of the field,” Isaiah 40:6) and its resilience.

The most famous poem about grass, by Walt Whitman, perhaps strikes the right elegiac note for the dinosaurs, who meet their doom tomorrow:

A child said, What is the grass? fetching it to me
with full hands;
How could I answer the child?. . . .I do not know what it
is any more than he.

I guess it must be the flag of my disposition, out of hopeful
green stuff woven.
Or I guess it is the handkerchief of the Lord,
A scented gift and remembrancer designedly dropped,
Bearing the owner’s name someway in the corners, that we
may see and remark, and say Whose?

Or I guess the grass is itself a child. . . .the produced babe
of the vegetation.

Or I guess it is a uniform hieroglyphic,
And it means, Sprouting alike in broad zones and narrow
zones,
Growing among black folks as among white,
Kanuck, Tuckahoe, Congressman, Cuff, I give them the
same, I receive them the same.

And now it seems to me the beautiful uncut hair of graves.
Tenderly will I use you curling grass,
It may be you transpire from the breasts of young men,
It may be if I had known them I would have loved them;
It may be you are from old people and from women, and
from offspring taken soon out of their mother’s laps,
And here you are the mother’s laps.

This grass is very dark to be from the white heads of old
mothers,
Darker than the colorless beards of old men,
Dark to come from under the faint red roofs of mouths.

O I perceive after all so many uttering tongues!
And I perceive they do not come from the roofs of mouths
for nothing.

I wish I could translate the hints about the dead young men
and women,
And the hints about old men and mothers, and the offspring
taken soon out of their laps.

What do you think has become of the young and old men?
What do you think has become of the women and
children?

They are alive and well somewhere;
The smallest sprouts show there is really no death,
And if ever there was it led forward life, and does not wait
at the end to arrest it,
And ceased the moment life appeared.

All goes onward and outward. . . .and nothing collapses,
And to die is different from what any one supposed, and
luckier.

Grassroots and fisticuffs

A couple of recent perspectives on Australopithecus and early Homo:

Tropical grasses and sedges commonly use the C4 pathway for photosynthesis, which evolved from the C3 pathway used by most plants, in response to lower atmospheric carbon dioxide levels. We can trace an isotopic signature in the teeth of animals that eat C4 plants (and animals that eat those animals). From about 3.5 Mya we see evidence of hominins eating more C4 foods: either grasses (maybe including underground parts) or animals that ate grasses. Paranthropus boisei seems to be a particular champion with the C4s. Chimpanzees, even when they live in open woodlands, mostly stick to eating fruits and leaves. By contrast, exploiting grassroots and sedges to varying degrees may be an important component of australopithecine adaptation to the savannah.

Not only do australopithecines have enlarged back teeth relative to great apes, but they also have smaller canines. Canines are mostly good for fighting, so it’s nice to think that canine reduction is a sign of our ancestors becoming more peaceful. Could be. But it’s possible they just switched fighting styles. David Carrier argues that australopithecine faces are built to withstand punches, starting from the time that australopithecine hands were capable of forming fists. This might go together with high levels of sexual dimorphism – males, the more violent sex, being bigger than females (but early hominin sexual dimorphism is a contentious subject.)

Grasses and gases for a cooling world

The standard sort of photosynthesis uses a so-called C-3 chemical pathway. But  maybe from 8-7 million years ago there’s an increasing proliferation of so-called C-4 plants. They use an alternative, more-efficient pathway to incorporate carbon from C02. C-4 plants evolved independently 45-60 times.

Tropical grasslands are mostly C-4. The profusion of grasses, herbivores, and carnivores on tropical savannahs will owe a lot to C-4 plants. This evolutionary transition is probably a sign that C02 levels are declining, and have reached a threshold where C-4 plants are favored.

Going back about 7 million years, C02 levels stood at maybe 1500 parts per million (ppm). (They were higher earlier in the Miocene.) Levels decline pretty steadily, leading to global cooling and eventual Ice Ages. But things never again reach the extremes of Snowball Earth 750 million years ago.

At the beginning of the Industrial Age C02 levels stood below 300 ppm. They went above 400 ppm last year.

Land of the thoats

There’s a great expansion in the diversity of horses in the mid-Miocene, especially horses adapted to grazing rather than browsing. The shift to grazing is goes on world wide among many different groups. In South America the big grazers are the liptoterns, ungulates not closely related to horses that evolve to look a lot like them, with high-crowned grazing teeth, single-toed hoofed feet and legs built for speed. (Edgar Rice Burroughs took the name thoat – what his characters rode around on on Barsoom/Mars — from one genus of liptotern, Thoatherium.)

thoat

Not from the horse family

We often think of evolution as a matter of organisms adapting to their environments, but when the environment is other organisms, each side may be chasing a moving target. Or sometimes the sides may reach an equilibrium. In the case of grazing animals, there’s a process of coevolution that goes on between grazers and grasses. Where grazers are active, the plants that survive are grasses, which keep leaves above the ground but grow from underground. And this works in the other direction: in moderately dry climates, grasses are more productive than taller brushy plants, so it’s when grasses take over that there’s enough food around for grazers – a mutually reinforcing cycle. With drier climates from the mid-Miocene on, grasslands and grazers get to be more and more important.

So a lot of the story of life on Earth is not just the appearance of this or that cool animal, but also the evolution of ecosystems. At the same time grasslands were spreading on land, for example, kelp forests were spreading in coastal oceans. We’ll see how important grasslands are in human evolution and history. And kelp forests, with their rich fish populations, might have been important too, as the highway that the earliest Americans followed along the Pacific coast to the New World.