Encyclopedia of Evolution.pdf - Online Reading Center

Piltdown man
found in slightly different forms (see convergence). All
of the enzymes involved in the first cycle (which produces
the acid) are used in other parts of the cell for other purposes.
The enzyme that captures the carbon dioxide, PEP
carboxylase, produces organic acids in all the cells of the
plant. Therefore the evolution of C 4 photosynthesis did not
require the origin of new chemical reactions but borrowed
them from other parts of the cell.
• CAM plants (which have Crassulacean Acid Metabolism)
also have the same two carbon fixation cycles, but the first
one (acid production) occurs at night, and the second one
(the Calvin cycle) occurs in the daytime. Like C 4 photosynthesis,
CAM appears to have evolved more than once, for
it is found in some species in several plant families (such
as the cactus, jade plant, and lily families) but not in their
common ancestors.
Both C4 and CAM pathways are adaptations to hot, dry
conditions:
• Rubisco, the enzyme that fixes carbon dioxide in the Calvin
cycle, is inhibited by oxygen. However, PEP carboxylase,
the enzyme that fixes carbon dioxide in the first cycle
of C4 photosynthesis, is not inhibited by oxygen. On a hot
dry day, leaves often close their pores (stomata) to prevent
excessive water loss. This causes a buildup of oxygen inside
the leaves. This oxygen buildup causes the photosynthesis
of C3 plants to slow down but does not inhibit the photosynthesis
of C4 plants. Therefore, in hot dry climates, C4 plants can continue making food under conditions that
would cause C3 plants to stop making food. Because the
carbon fixation in C4 plants has two cycles rather than just
one, it is more expensive; but in hot dry climates the extra
expense is worth the cost. However, in cool moist climates,
the expense is not worth the cost. Therefore C4 plants are
relatively more common in hot dry climates (usually grasslands)
and C3 plants in cool, moist climates. C4 photosynthesis
has also evolved in some aquatic plants, because
carbon dioxide is often scarce in water. The PEP carboxylase
helps them obtain more carbon dioxide than would
rubisco acting alone.
• CAM plants open their pores and absorb carbon dioxide
from the air at night, when conditions are relatively cool
and moist. They make it into acid and store the acid until
dawn. Then they close their pores and use the acid as a
source of carbon dioxide from which the Calvin cycle
can produce sugar during the day. This has the advantage
of allowing the CAM plants to open their pores at night
rather than during the day, but it has a cost: They have
to store the acid in their tissues. Most CAM plants are
succulents (such as cactuses, jade plants, and agaves). As
with C4 photosynthesis, CAM is worth the cost in hot, dry
climates (usually deserts) but not worth the cost in cool,
moist climates.
The fact that most C 4 plants are grasses (which did not
become abundant until the middle of the Cenozoic era),
and that C 4 has evolved several times, suggests that it is of
recent origin. CAM appears to also be of recent origin.
Before oxygen gas was abundant in the atmosphere of
the Earth, ultraviolet radiation bombarded the surface of the
planet. This would probably have killed most life-forms at or
near the surface of the ocean. Most scientists believe that the
first life-forms evolved below the ocean surface, perhaps at
deep sea vents (see origin of life). Oxygen gas produced by
photosynthesis would have overcome the problem of ultraviolet
radiation and allowed photosynthetic organisms to grow
at or near the surface. The obvious problem is, what were the
photosynthetic organisms doing when the only safe place for
them to live was deep in the ocean? The great sensitivity of
anoxic photosynthetic bacteria to light was mentioned above.
In 2005 scientists announced the discovery that anoxic green
sulfur bacteria could carry out photosynthesis over 200 feet
(80 m) below the surface of the Black Sea, under conditions
that appeared to human observers to be totally dark. Sensitive
measurements indicated that the volcanic vents in this
region produced light in the visible range, and that the bacteria
could use this faint light for photosynthesis. These bacteria
were also, unlike other green sulfur bacteria, not killed by
oxygen gas. In two ways—their ability to use very faint light
and their ability to tolerate oxygen—these bacteria may represent
what the first photosynthetic cells were like.
Further Reading
Bohannon, John. “Microbe may push photosynthesis into deep
water.” Science 308 (2005): 1,855.
Nasa Astrobiology Institute. “Terrestrial Powerhouses.” Available
online. URL: http://www.nai.arc.nasa.gov/news_stories/news_
detail.cfm?article=old/powerhouses.htm. Accessed May 1, 2005.
Whitmarsh, John, and Govindjee. “The Photosynthetic Process.”
Available online. URL: http://www.life.uiuc.edu/govindjee/paper/
gov.html#32. Accessed May 1, 2005.
Piltdown man Piltdown man was a fraudulent fossil
human that was planted in a gravel quarry in England in
1911. The perpetrator/s of the hoax, which was revealed in
1953, were never definitely identified, although the person
who had greatest opportunity was Charles Dawson, a lawyer
and amateur archaeologist who was involved in other fraudulent
schemes. At that time, the only other fossil humans,
aside from those that were clearly in the modern human species
(see Cro-Magnon), were the Neandertals in mainland
Europe and Java man (see Homo erectus; Dubois,
Eugène). Throughout the early period of the study of human
evolution, the question persisted: Which came first, upright
posture or a large brain? The Neandertal and Java man skeletons
suggested that apelike features of the skull persisted even
after erect posture had evolved in the human lineage. Piltdown
man was, for four decades, accepted as evidence that
brain expansion may have preceded the evolution of the rest
of the skeleton. Also, this ancient expansion of brain capacity
just happened to occur in what is now England, which at the
beginning of the 20th century still considered itself the most
intellectually advanced nation in the world.
In 1911 Dawson visited the Piltdown gravel quarry near
his home and asked the quarrymen to be on the watch for fos-