Galapagos 2005 - 2


Day Two : Evidence for Evolution

The first session the following morning began with a lecture on the
origins of life by Antonio Lazcano, President of the International
Society for the Study of the Origins of Life and a scientist at the
Universidad Autónoma de México, who theorized that there were three
sources for the primordial soup: a reducing atmosphere from volcanic
outgassing, high-temperature submarine vents and fumaroles, and space —
the 4.6 billion-year-old Murchison meteorite, discovered in Australia in
1969, for example, was loaded with amino acids, aliphatic and aromatic
hydrocarbons, hydroxy acids, purines, pyrimidines, and other chemical
building blocks of life. “The evidence strongly suggests that prior to
the origin of life the primitive Earth already had many different
catalytic agents, polymers with sequences of nucleotides, and
membrane-forming compounds,” Lazcano concluded. This prebiotic soup led
to a catalytic and replicative RNA world, which led to the DNA world of
today.

UCLA paleobiologist William Schopf began his commentary on Lazcano’s
lecture by quoting the U.S. Secretary of Defense Donald Rumsfeld:

(quote)
There are known knowns. There are things we know we know. We also
know there are known unknowns. That is to say, we know there are some
things we do not know. But there are also unknown unknowns, the ones we
don’t know we don’t know.
(end quote)

Translating Rumsfeld, Schopf asked: “What do we know? What are the
unsolved problems? What have we failed to consider?” Schopf answered:

(quote)
We know the overall sequence of life’s origin, from CHONSP (carbon,
hydrogen, oxygen, nitrogen, sulfur, phosphorus), to monomers, to
polymers, to cells; we know that the origin of life was early,
microbial, and unicellular; and we know that an RNA world preceded
today’s DNA-protein world. We do not know the precise environments of
the early earth in which these events occurred; we do not know the exact
chemistry of some of the important chemical reactions that led to life;
and we do not have any knowledge of life in a pre-RNA world.
(end quote)

As for what we have failed to consider, Schopf suggested that the “‘pull
of the present’ makes it extremely difficult for us to model the early
earth’s atmosphere and the biochemistry of early life.”

In the discussion period University of Massachusetts theoretical
biologist Lynn Margulis, in her inimitable rapid-fire style, hit Lazcano
with a point-blank question: “In your opinion what came first, cells or
the RNA world?” Lazcano answered:

(quote)
If you define a cell as a membrane-enclosed system, then
lipids-enclosed systems assisted in the polymerization of molecules,
which led to RNA. Lipids and cells came first, then the RNA world.
(end quote)

Next up was Mikhail Fedonkin, head of the Laboratory of the Precambrian
Organisms at the Paleontological Institute in Moscow, with a lecture on
evolution in the Proterozoic and Archean Eons, which extend back to more
than 3.6 billion years ago and cover the first microfossils and
stromatolite fossils. Fredonkin suggested that a fall of global
temperatures and the oxygenation of the biosphere due to photosynthesis
played a major role in the dramatic change in the availability of heavy
metals that he believes were crucial in the metabolic processes that led
to the evolution of complex life. This metal-rich environment served as
a catalyst: “Over 70 percent of known enzymes contain metal ions as a
cofactor of an active site. Fast catalyzed reactions segregated life
first dynamically and then structurally from the mineral realm.” Once
prokaryotes gave rise to eukaryotes (through symbiogenesis — Fedonkin
supports Margulis’ theory of the origins of modern cells), life was off
and running, exploding in the Cambrian with complex hard-bodied organisms.

Stefan Bengtson, from the Swedish Museum of Natural History, commenting
on Fedonkin, asked “Why did the build-up to the Cambrian ‘explosion’
take so long?” Noting that 99.99999% of all living species who ever
lived have gone extinct, Bengston reflected: “We do not know because we
have nothing else to go on. Life is an evolutionary bush, not an
evolutionary tree, but our data based on extant life induce us to prune
the bush into a tree, so we need more data.”

Richard Fortey from the British Museum of Natural History was next in
the lineup, in which he discussed the evidence of evolution in the
Phanerozoic (from 542 million years ago till the present), emphasizing
the importance of mass extinction events in resetting the direction of
evolution, the importance of evolutionary arms races in driving
morphological innovation, the relationship of climate change and
changing geography to evolutionary change, and the extent to which
evolution can be described as directional. With half a billion years of
a solid fossil record, Fortey said we can track the evolutionary periods
of creativity and crises. Stephen Jay Gould’s Wonderful Life stimulated
a lot of new ideas about the Cambrian explosion of life, he continued,
and it soon became clear that there were a huge variety of organisms
difficult to classify, such as those in the Burgess Shale. But there are
a number of Cambrian fossil beds, such as in China, where important
phyla such as Chordata evolved. “But what does all this diversity mean?”
Fortey asked.

(quote)
There are today 30 living phyla. In the Cambrian, some claim that
there were as many as 100 phyla, but the evidence does not support this.
We now believe that morphological diversity did not explode as much as
Gould originally suggested, although the explosion in evolutionary
experimentation was real. By the time we get to the Cambrian, like at
the Burgess Shale, the systems are very complex, such as trilobite eyes.
Evolution was experimenting with many wondrous varieties, such as all
the armor on the heads of trilobites.
(end quote)

Interestingly, despite the impact of the five biggest mass extinctions
(Ordovician 439 Ma, Devonian 367 Ma, Permian 245 Ma, Triassic 208 Ma,
Cretaceous 65 Ma), many organism groups passed through all of these
extinction episodes safely, such as the cockroach. “What is amazing is
not only the extent of loss, but how fast life bounces back,” Fortey
concluded.

In the subsequent discussion session, Bill Schopf asked all the speakers
the Gouldian question: if we reran the tape of life would we end up with
something like what we have today? The collective response was that it
depends on how the question is defined, as in “what do you mean by
‘something like’?” There is evolutionary convergence, so clearly some
things would be preserved (like eyes and wings). The experiment has been
run in that sense. Fortey said that such “what if” questions are
meaningless, but that’s not true, since counterfactual history is a
legitimate form of reasoning about cause and effect relationships.

Next on the roster was Peter Gogarten, a professor of molecular and cell
biology at the University of Connecticut, who asked “Is the ‘Tree of
Life’ a Tree?” When we are talking about prokaryote evolution,
horizontal gene transfer between organisms allows us to understand
genealogical relationships, he explained.

(quote)
Over long periods of time gene transfer makes organisms existing in
the same environment more similar to one another. This is most clearly
seen in the case of organisms that live in environments that are
otherwise inhabited by distant relatives only.
(end quote)

Thus, Gogarten concluded,

(quote)
the boundaries between prokaryotic species are fuzzy. Therefore the
principles of population genetics need to be broadened so that they can
be applied to higher taxonomic categories.
(end quote)

Margaret Riley, a colleague of Margulis at the University of
Massachusetts-Amherst, provided the commentary on Gogarten’s talk,
suggesting that we need a modification of Ernst Mayr’s definition of a
species to accommodate microbes. Mayr defined a species as: “A group of
actually or potentially interbreeding natural populations reproductively
isolated from other such populations.” The problem with applying this
definition to microbes is that separate species are not truly
reproductively isolated, and yet they still retain distinct features
that keep them phenotypically apart. “Although horizontal gene transfer
can and does occur, it does not obliterate the phenotypic groupings of
organisms,” Riley concluded.

Australian botanist and itinerant surfer Geoff McFadden, from the
University of Melbourne, lectured next on “Protists and Cellular
Phenomena in Evolution,” opening with the semi-disgusting story of how
Anton van Leeuwenhoek discovered the first protists by training his hand
made microscope on his own diarrhoeal stool. Whatever it takes to get
the data, I suppose, but I was glad that dinner was still hours away.
Darwin apparently ignored protists, but Ernst Haeckel included them in
his comprehensive tree of life, and Constantin Mereschkowsky was the
first to appreciate the significance of protists in early eukaryotic
evolution. A.F.W. Schimper noted that chloroplasts in plant cells very
much resembled cyanobacteria, but the ultimate theoretical model was
provided by Lynn Margulis: the key step was the endosymbiosis of
cyanobacteria within a phagotrophic eukaryotic host, a process she calls
symbiogenesis. In primary endosymbiosis, 1,000 genes were acquired by
the nucleus from the incorporated cyanobacteria. In secondary
endosymbiosis, there was a second round of gene transfer in which the
eukaryote cell engulfs another plastid-containing eukaryote.
Creationists and Intelligent Design theorists like to inquire how
information can increase in a world filled with entropy and the decay of
information. Symbiogenesis is one answer — incorporating the genome of
other organisms. Lynn Margulis would have much more to say on this in
her lecture the last day.

One of the best talks of the conference was delivered by the U.C.
Berkeley paleoanthropologist Timothy White, in which he opened with a
prediction made by Stephen Jay Gould in the late 1980s: “We know about
three coexisting branches of the human bush. I will be surprised if
twice as many more are not discovered before the end of the century.” A
glance at the extant fossil record looks like Gould was right. There are
at least two dozen fossil species in six million years of hominid
evolution. But the bush is not so bushy, says White. The problem lies in
the difference between “lumpers” and “splitters” in species
classification, and the social pressures to publish extraordinary new
discoveries. If you want to get your fossil find published in Science or
Nature, and you want the cover illustration, you cannot conclude that
your fossil is yet another Australopithicus africanus, for example. You
better come up with an interpretation indicating that this new find you
are revealing for the first time to the world is the most spectacular
discovery of the last century and that it promises to overturn hominid
phylogeny and send everyone back to the drawing board to reconfigure the
human evolutionary tree. Training a more skeptical eye on many of these
fossils, however, shows that many, if not most of these fossils belong
in already well-established categories. White says that the specimen
labeled Kenyanthropus platyops, for example, is very fragmented and is
most likely just another Australopithicus africanus. “Name diversity
does not equal biological diversity,” White elucidated.

White then concluded his talk with a fascinating discussion of the
recent discovery of fossil dwarf humans on Flores Island in the Malay
Archipelago, located on the outside of Wallace’s Line, meaning that even
during the last ice age they could only have gotten there by boat.
(White did note, however, that after last December’s tsunami people were
rescued from large floating rafts of natural debris, so it is possible
that the founding population of Flores rafted there by accident and not
design.) Found in Liang Bua cave, the type specimen of Homo floresensis
was dated at 18,000 years old, meaning that they had to be modern humans
because all other hominid species had long ago gone extinct. But with a
cranial capacity of only 300cc — about the same size as that of Lucy and
modern chimpanzees — this means that they were able to fashion complex
tools (and possibly boats) with tiny brains; the implication is that
brain architecture, not size, is what counts for creating higher
intelligence. A second published specimen put to rest the pathology
hypothesis that Homo floresensis was a microcephalic human. The best
evidence, says White, points to insular dwarfing, a rapid punctuation
event out of Homo sapiens that led to a shrinkage of these isolated
people. Such dwarfing effects can be seen on this and other islands,
where large mammals get smaller (like the dwarf elephant), and small
reptiles get larger (like the Komodo Dragon). The chances of any living
members of this species still existing in the hinterlands of Flores are
extremely remote, but some observers have noted that the indigenous
peoples of Flores recount a myth of small hairy humans who descend from
the highlands to steal food and supplies.

University of Cambridge professor Peter Forster, an expert in
archaeogenetics, followed Tim White by showing how prehistoric human
migrations can be traced by mitochondrial DNA (mtDNA) through the
maternal line of modern humans. The mtDNA samples are taken through
saliva cheek swabs, then dried to prevent molding before analysis is
conducted in the lab. The process was first done in 1981, using the
placenta of a woman in a maternity ward, and has since become a mainstay
of researchers in this field. Forster outlined our migrational history
over the past 200,000 years as follows: Between 190,000–130,000 years
ago, a single female known formally as the “mitochondrial coalescent”
but dubbed “mitochondrial Eve,” gave rise to every living human today.
Between 80,000–60,000 years ago, a large population from the center of
Africa migrated to all areas of Africa, as well as the area of
present-day Saudi Arabia. This migration may have taken two routes, a
northern one up the Nile and around the Red Sea, and a southern one
across the narrow straight which, during the last ice age would have
only been five kilometers across (Forster thinks the latter the most
likely route). Between 60,000–30,000 years ago there was a great
migration to Southeast Asia, Northern Asia, and Europe. Between
30,000–20,000 years ago, people spread throughout the rest of the world,
including Australia, and between 20,000–15,000 years ago they migrated
into North America, making their way into South America between
15,000–2,000 years ago. The final migration over the past 2,000 years
saw the settlement of the Pacific islands.

The next lecture would have sent Darwinian fundamentalists into
skeptical paroxysms, as Leticia Aviles, a zoologist at the University of
British Columbia, summarized the evidence for “multilevel selection.”
Darwinian fundamentalists (an intentionally pejorative term coined by
Stephen Jay Gould) believe that the individual organism is the sole
target of natural selection. Aviles said that below the individual,
selection may occur at the level of genes, chromosomes, organelles, and
cells. Above the individual, selection may occur at the level of social
groups, demes, species, and multispecies communities. In that sense,
Aviles said, “individual” depends on the frame of reference. She then
applied multilevel selection to research on sex ratios, cooperation
among non-relatives, and multicellularity. Social spiders are an example
of group selection, Aviles continued. And sex ratios that depart from
1:1 cannot be accounted for by inbreeding alone, so group selection is
here invoked. Likewise, the equilibrium of sex ratios is explained by
both within-group selection and between-group selection. “When
cooperation is not costly, groups, grouping, and cooperation evolve
readily. But with increasing costs of cooperation, levels of cooperation
decrease.”

The highlight of the second day was the lecture by the husband and wife
team Peter and Rosemary Grant, both from Princeton University, made
famous by Jonathan Weiner in his 1994 book The Beak of the Finch. Every
year for the past three decades the Grants have parked themselves on
Daphne Major, a tiny volcanic plug of an island 120 meters high and a
kilometer long to study Darwin’s finches and the process of speciation.

Three million years ago an ancestral group of finches flew out to the
Galapagos during a time of very active plate tectonics and the creation
of the island archipelago. When this founder population arrived it
encountered a very different environment from the one we see today:
there were only five islands and the temperatures were much higher. Over
the last three million years of fluctuations in global temperatures,
there has been an overall net cooling of the islands. But when these
little finches arrived 2.75 million years ago there was a permanent El
Niño and the islands were warm and wet, during which there was an
explosion of speciation. First came the warbler finch, then the tree
finch (of which there are now five species) and then the ground finch
(of which there are now six species). Following Ernst Mayr’s theory of
allopatric speciation (where a founder daughter population breaks away
from the parental population), the first finches landed on San
Cristóbal, then migrated to Espanola, then to Floreana, then to Santa
Cruz, and finally made their way back to San Cristóbal. Along the
journey the finches adapted to local conditions. Finches in highlands
developed larger beaks to break hard beetles and seeds. Finches in
lowlands evolved smaller beaks for eating small seeds and succulents. As
an opportunistic species, some of these finches also ate sea turtle eggs
and sucked the blood from blue-footed boobies. Different adaptations to
different islands lead to speciation.

The strongest environmental factor the Grants have observed is the
rainfall pattern over 30 years on Daphne Major. Arriving in 1973, the
Grants immediately witnessed a draught that wiped out 85 percent of the
population of two species of finches (the ground finch Geospiza fortis
and the cactus finch Geospiza scandens). From 1975 to 1978 there was
almost no rainfall and natural selection operated rapidly to change beak
size. In 1983, an El Nino rainfall produced an abundance of plants and
trees and cactus fruit, all covered by vines. Two years after the El
Nino event, the island dried out and the large seeds were replaced by
small seeds, leading to a favoring of small pointy beaked birds. Beak
shape, beak size, and body size all changed in parallel. The Grants
summarized four lessons they learned about natural selection on Daphne:

1. It is an observable, measurable process in a natural environment.
2. It oscillated in direction.
3. It occurs when the environment changes.
4. It has evolutionary significance.

The Grants have made another important observation on a reproductive
isolating mechanism in finches: song. Song is learned during a short
sensitive period early in the life of a finch (between days 10 and 30),
while still in the nest and being fed by their fathers. Only the males
sing. A few learn variations on the song. Rosemary recounted an
endearing story about a finch who got a cactus spine stuck in its throat
that made its song more croaky; his sons subsequently learned the new
croakier song, as did their sons, and so on through the generations, a
clear example of a meme.

The Grants are heroes among evolutionary biologists, and their mere
presence lifted the conference to a higher status, which was
reciprocated the final day of the conference when they were awarded
honorary doctorates from the Universidad San Francisco de Quito.
.

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