The fossil record is chock-full of ground-dwelling sloths ranging
from medium-sized to mammoth (literally). But these days, there are only
a few small sloths, and they all live in trees. Researchers
reconstructing sloth evolution found that extinct sloths developed large
body sizes at an amazing rate. Existing sloths are the black sheep of
the sloth family: not only do they not reflect the overall evolutionary
trends of the group, but they also obscure the strong (but long gone)
signal in the fossil record of ever increasing body size.
Sloths were incredibly diverse in the past, with more than 50 known
species distributed among eight families. Animals in at least half of
those families weighed over 1,000 kilograms. Megatherium americanum grew to the size of an elephant, and the claws of Eremotherium eomigrans
were about a third of a meter long. They showed up around 50 million
years ago, but multiple megafaunal extinction events—from 2.6 million to
11,700 years ago—cut their diversity down by about 90 percent.
All six sloth species today belong to just two genera, Bradypus and Choloepus, each in their own family: four three-toed sloths in Bradypodidae, and two two-toed sloths in Megalonychidae.
The two lineages may seem similar, but they diverged from one another
about 30 million years ago. So oddly enough, they must have
independently evolved their small size—no more than six kilograms—and
arboreality, or tree-living.
Using a data set of all 57 species of known living and fossil sloths, a trio of researchers led by John Finarelli of University College Dublin
examined changes in body mass through sloth evolution. The team found a
clear trend for the evolution of larger and larger body sizes through
time—and they evolved at an extremely fast rate. The work was published in BMC Evolutionary Biology this week.
Megatherium’s family saw an average mass increase of 129
kilograms per million years—one of the fastest rates of body size
evolution known for mammals. Even the family Megalonychidae (which includes today’s two-toed sloths) had an average body mass increase of 2.6 kilograms per million years.
The findings suggest that environmental conditions at the time, such
as climate or species competition, must have really favored larger body
sizes. Whatever the cause of their eventual decline, only
small, arboreal sloths survived these events, completely reversing the
millions-year-old trend towards larger and larger sloths, the BMC blog reports.
“If we ignore the fossil record and limit our studies to living
sloths, as previous studies have done, there's a good chance that we'll
miss out on the real story and maybe underestimate the extraordinarily
complex evolution that produced the species that inhabit our world,"
says study co-author Anjali Goswami of Univeristy College London in a news release.
And sloths may not be the only group where modern-day species are
unrepresentative of overall evolutionary trends. The diversity of
hyenas, elephants, and rhinos, for example, are only a fraction of what
it was in the past.
At fifteen he received commissions from Cardinal Richelieu, in the execution of which he displayed an ability which obtained the generous commendations of Nicolas Poussin, in whose company Le Brun started for Rome in 1642
In 1663, he became director of the Académie royale de peinture et de sculpture, where he laid the basis of academicism and became the all-powerful, peerless master of 17th-century French art.
He used his artistry to compare human and animal faces, later inspiring Charles Darwin to write The Expression of the Emotions in Man and Animals.(1872).
However, Darwin viewed this work as a cornerstone of his
evolutionary theory— the means of demonstrating once and for all that
Man was not a separate and divinely created species but no different
from other animals.
The result was a study of expression that tried to
identify specific mental and emotional states as well as their
corresponding expressions.
[Physiognomy,
literally the "knowledge of nature," relates to the assessment
of human character through study of physical features. This concept
of an inherent concordance of body and soul harks back to Antiquity,
and regained currency in 17th-century thought. [The goal of
physiognomy is to judge character according to features of
the face.]
A number of thinkers have considered how the face reflects the workings
of the mind, and ultimately the soul, and human nature. For many
centuries the study of Physiognomy was tied up with astrology. Notions
of the relationship between individual appearance and inner character,
was present in early Greek culture.
By the fourth century, the
philosopher Aristotle made frequent reference to Physiognomy.
Aristotle’s considered that a man’s mental nature, as well as his
passing emotional state, was revealed by his anatomic form and the play
of the muscles.
The study of Physiognomy became an integral part of the
physiologically based medical practice and those who study physiognomics
were “physicians” who tried to interpreted human character and moral
tendencies.
Analogies were drawn between human expressions (pride,
humility, cowardice, bravery) and animals’ facial features. In the
ancient Greek world view, the peacock was proud, the hare timorous, the
lion brave.]
LeBrun studied the lines linking different points
of the head in a complex geometry which revealed the
faculties of the spirit or character.
Thus, the angle formed
by the axis of the eyes and the eyebrows could lead to
various conclusions, depending upon whether or not this
angle rose toward the forehead to join the soul or descanted
toward the nose and mouth, which were considered to be
animal features.
Le Brun of course drew on various contemporary sources, both graphic
and philosophical, two of which come immediately to mind:
G. B. Della Porta's De Humana
Physiognomia had been translated into French in 1655-56,
and certain of Le Brun's drawings relate closely to his Italian
predecessor,
and more importantly,
René Descartes
published his Passions de l'Ame in 1649, identifying the
seat of the soul in the pineal gland, located at the center of
the brain.
Le Brun took this as a starting point
for his treatise. Elaborating a geometry that links the soul to
the senses, in witness of faculties and character, he studied
the lines relating the different features of the head. The angle
thus formed by the eyes and the browline is here indicative of
the individual's aspirations: directed upward they would be lofty
and spiritual; downward to nose and mouth, the animal elements
of base sensory experience, they would be vile (see Plate 1).
Thus the position and the conformation
of the eyes greatly help to read the dominant passions (Cf. the
demonstration in Pl. 12, where Le Brun bestows human eyes on the
lion and the horse).
As related by Henri Testelin (in his
3ème Discours Académique),
"The affections of the soul follow
the temperament of the body and the external marks are certain
signs of the affections of the soul, that we know in the form
of each animal, its mores and its complexion...
The difference between the human face
and that of the brutes is that man has eyes located on the same
line that traverses straight to the nerve of the ears, which
leads to the sense of hearing; the brutish animals to the contrary
have the eye drawing downward to the nose, following their natural
affections."
Testelin follows up on the geometry in
animals, stating:
"It was demonstrated by a triangle
that the sense impressions [i.e. feelings] of animals are borne
from the nose to the ear, and from there to the heart, of which
the bottom line comes to close up the angle with that of the
nose; and that when this line traverses the whole eye, and that
on the bottom passes through the mouth, this marks the animal
as fierce, cruel, carnivorous.
There is also a small triangle of which
the point is at the outside corner of the eye; whence the line
following the contour of the upper eyelid forms an angle with
that coming from the nose. When the point of this angle meets
toward the brow, it is a mark of mind ("esprit"),
as is seen in elephants, camels, and monkeys; and if this angle
falls on the nose, this marks stupidity and imbecility, as in
asses and sheep..."
Here Le Brun develops the triangular
geometry relating to what he apparently considered to be significant
examples of animals, along with their human equivalents. Drawing
on Testelin, Morel d'Arleux comments thus:
"He [Le Brun] supposed an equilateral
triangle, of which the base AB, passing through the inside of
the eye at E, finding itself cut off at point A, at the tip of
the nose and at point B, either the tympanum of the ear or the
base of the horns...
If the animal was carnivorous, he drew
a parallel to the side BC of the triangle that ran through the
inner corner of the eye at E, cutting more or less across the
mouth at G, according to the voracity, but that would be found
well inside if it were herbivorous . This same parallel extended
to the brow was to strike the sign of strength, indicated by
a greater elevation of this part and denoted at the same time
the degree of an animal's courage.
The line HI, starting from the outer
corner of the eye next to the upper lid and extending to the
brow, reveals the degree of an animal's sagacity by its elevation,
of mansuetude by its tending to the horizontal, of meanness or
abasement by an inclination on the nose.
The outer parallel KL, drawn to the
base AB of the triangle ACB and grazing the highest elevation
of the brow, comes to bear on the preceding observation, leaving
more or less room between it and the muzzle according to whether
or not the animal is endowed with intelligence."
“Even after thirteen thousand years, avocado is clueless that the great mammals are gone.”
In any market economy, it’s common sense that as soon as the consumer for a certain product ceases to exist, the product itself becomes moot and soon vanishes from stores. In nature, however — or market ecology, if you will — that need not necessarily be the case. In the altogether fascinating
The Ghosts of Evolution: Nonsensical Fruit, Missing Partners, and Other Ecological Anachronisms (public library), popular science writer and evolutionary biology champion Connie Barlow builds on the work of renowned ecologists Dan Janzen and Paul Martin, who in 1982 published a provocative paper arguing that many of the fruits and nuts found in Central American forests today evolved to be eaten by animals that have been extinct for thousands of years. Barlow explores the curious anachronistic existence of these species, ranging from papayas to persimmons to ginkgo biloba, and even coffee.
But as an avid aficionado of the avocado, I was especially taken with its particular story: Since fruits propagate by seeds, their progeny doesn’t grow far from the tree, as the proverb goes; their only chance of spreading their seeds across the land, then, are the animals who eat the fruit, along with its seeds, then “plant” those elsewhere when they poop.
The avocado’s abnormally giant seed presents anything from a severe digestive hazard to a death sentence for contemporary earthly species but, apparently, avocados coevolved with ground sloths and were originally eaten by gomphothere — elephant-like creatures that lived during the Miocene and Pliocene, between 12 million and 1.6 million years ago, who happily reaped the fruit with their hefty trunks, crunched them with their massive teeth, and passed the seeds comfortably through their oversized digestive tract.
Relic of a ghost: An avocado fruit and seed paired with the tooth of its missing partner in evolution, the gomothophere Cuvieronius.
The problem, of course, is that gomphothere no longer roam the Earth — and yet avocados still exist. Barlow writes:
Avocado’s strategy for propagation made a great deal of sense throughout the long life of its lineage — until the present moment. Even after thirteen thousand years, avocado is clueless that the great mammals are gone. For the avocado, gomphothere and ground sloths are still real possibilities. Pulp thieves like us reap the benefits. Homo Sapiens will continue to mold the traits of the few species of genus Persea it prefers. Ultimately, however, wild breeds will devolve less grandiose fruits, or else follow their animal partners into extinction.
In this fascinating short video for PBS, Joe Hanson of It’s Okay To Be Smart explains the avocado’s curious fate, along with those of its brethren of ecological anachronism:
There's
a fundamental mystery at the core of our evolution. No, it's not how we
went from fuzzy shrews to humans — it's how bacteria made the jump from
single-celled existence to something more complex. The weird part is
that evolutionary jump only happened once.
Over at Nautilus,
Ed Yong has a terrific essay about that moment, roughly 2 billion years
ago, when bacteria made an incredible evolutionary leap. It put them on
a path that eventually led to the evolution of complex, multicellular
animals like us. But how the hell did it happen.
Yong
writes about a new theory that could shed light on the most important
missing link in our history as animals. Here's how he starts:
At
first glance, a tree could not be more different from the caterpillars
that eat its leaves, the mushrooms sprouting from its bark, the grass
growing by its trunk, or the humans canoodling under its shade.
Appearances, however, can be deceiving. Zoom in closely, and you will
see that these organisms are all surprisingly similar at a microscopic
level. Specifically, they all consist of cells that share the same basic
architecture.
These cells contain a central nucleus—a command
center that is stuffed with DNA and walled off by a membrane.
Surrounding it are many smaller compartments that act like tiny organs,
carrying out specialized tasks like storing molecules or making
proteins. Among these are the mitochondria—bean-shaped power plants that
provide the cells with energy.
This combination of features is
shared by almost every cell in every animal, plant, fungus, and alga, a
group of organisms known as "eukaryotes."
Bacteria showcase a
second, simpler way of building a cell—one that preceded the complex
eukaryotes by at least a billion years. These "prokaryotes" always
consist of a single cell, which is smaller than a typical eukaryotic one
and bereft of internal compartments like mitochondria and a nucleus.
Even though limited to a relatively simple cell, bacteria are impressive
survival machines. They colonize every possible habitat, from
miles-high clouds to the deep ocean. They have a dazzling array of
biological tricks that allow them to cause diseases, eat crude oil,
conduct electric currents, draw power from the Sun, and communicate with
each other.
Still, without the eukaryotic architecture, bacteria
are forever constrained in size and complexity. Sure, they have their
amazing skill sets, but it's the eukaryotes that cover the Earth in
forest and grassland, that navigate the planet looking for food and
mates, that build rockets to Mars.
The transition from the classic
prokaryotic model to the deluxe eukaryotic one is arguably the most
important event in the history of life on Earth. And in more than 3
billion years of existence, it happened exactly once.
Life is full
of complex structures that evolve time and again. Individual cells have
united to form many-celled creatures like animals and plants on dozens
of separate occasions. The same is true for eyes, which have
independently evolved time and again. But the eukaryotic cell is a
one-off innovation.
Bacteria have repeatedly nudged along the path
towards complexity. Some are very big (for microbes); others move in
colonies that behave like single, many-celled creatures. But none of
them have acquired the full suite of crucial features that define
eukaryotes: large size, the nucleus, internal compartments,
mitochondria, and more. As Nick Lane from University College London
writes, "Bacteria have made a start up every avenue of eukaryotic
complexity, but then stopped short." Why?
It is not for lack of
opportunity. The world is swarming with countless prokaryotes that
evolve at breathtaking rates. Even so, they were not quick about
inventing eukaryotic cells. Fossils tell us that the oldest bacteria
arose between 3 and 3.5 billion years ago, but there are no eukaryotes
from before 2.1 billion years ago. Why did the prokaryotes remain as
simple cells for so damn long? There are many possible
explanations, but one of these has recently gained a lot of ground. It
tells of a prokaryote that somehow found its way inside another, and
formed a lasting partnership with its host. This inner cell—a
bacterium—abandoned its free-living existence and eventually transformed
into the mitochondria. These internal power plants provided the host
cell with a bonanza of energy, allowing it to evolve in new directions
that other prokaryotes could never reach.
If this story is true,
and there are still those who doubt it, then all eukaryotes—every flower
and fungus, spider and sparrow, man and woman—descended from a sudden
and breathtakingly improbable merger between two microbes.
At first glance, a tree could not be
more different from the caterpillars that eat its leaves, the mushrooms
sprouting from its bark, the grass growing by its trunk, or the humans
canoodling under its shade. Appearances, however, can be deceiving. Zoom
in closely, and you will see that these organisms are all surprisingly
similar at a microscopic level. Specifically, they all consist of cells
that share the same basic architecture.
These cells contain a
central nucleus—a command center that is stuffed with DNA and walled off
by a membrane. Surrounding it are many smaller compartments that act
like tiny organs, carrying out specialized tasks like storing molecules
or making proteins. Among these are the mitochondria—bean-shaped power
plants that provide the cells with energy.
This combination of
features is shared by almost every cell in every animal, plant, fungus,
and alga, a group of organisms known as “eukaryotes.”
Bacteria
showcase a second, simpler way of building a cell—one that preceded the
complex eukaryotes by at least a billion years. These “prokaryotes”
always consist of a single cell, which is smaller than a typical
eukaryotic one and bereft of internal compartments like mitochondria and
a nucleus. Even though limited to a relatively simple cell, bacteria
are impressive survival machines.
They colonize every possible habitat,
from miles-high clouds to the deep ocean. They have a dazzling array of
biological tricks that allow them to cause diseases, eat crude oil,
conduct electric currents, draw power from the Sun, and communicate with
each other.
Still, without the eukaryotic architecture,
bacteria are forever constrained in size and complexity. Sure, they have
their amazing skill sets, but it’s the eukaryotes that cover the Earth
in forest and grassland, that navigate the planet looking for food and
mates, that build rockets to Mars.
The transition from the
classic prokaryotic model to the deluxe eukaryotic one is arguably the
most important event in the history of life on Earth. And in more than 3
billion years of existence, it happened exactly once.
Life is
full of complex structures that evolve time and again. Individual cells
have united to form many-celled creatures like animals and plants on
dozens of separate occasions. The same is true for eyes, which have
independently evolved time and again. But the eukaryotic cell is a
one-off innovation.
Bacteria have repeatedly nudged along the
path towards complexity. Some are very big (for microbes); others move
in colonies that behave like single, many-celled creatures. But none of
them have acquired the full suite of crucial features that define
eukaryotes: large size, the nucleus, internal compartments,
mitochondria, and more. As Nick Lane from University College London
writes, “Bacteria have made a start up every avenue of eukaryotic
complexity, but then stopped short.” Why?
The transition is arguably the most important event in the history of life on Earth.
It
is not for lack of opportunity. The world is swarming with countless
prokaryotes that evolve at breathtaking rates. Even so, they were not
quick about inventing eukaryotic cells. Fossils tell us that the oldest
bacteria arose between 3 and 3.5 billion years ago, but there are no
eukaryotes from before 2.1 billion years ago. Why did the prokaryotes
remain as simple cells for so damn long?
There are many possible
explanations, but one of these has recently gained a lot of ground. It
tells of a prokaryote that somehow found its way inside another, and
formed a lasting partnership with its host. This inner cell—a
bacterium—abandoned its free-living existence and eventually transformed
into the mitochondria. These internal power plants provided the host
cell with a bonanza of energy, allowing it to evolve in new directions
that other prokaryotes could never reach.
If this story is true,
and there are still those who doubt it, then all eukaryotes—every
flower and fungus, spider and sparrow, man and woman—descended from a
sudden and breathtakingly improbable merger between two microbes.
They
were our great-great-great-great-...-great-grandparents, and by becoming
one, they laid the groundwork for the life forms that seem to make our
planet so special. The world as we see it (and the fact that we see it
at all; eyes are a eukaryotic invention) was irrevocably changed by that
fateful union—a union so unlikely that it very well might not have
happened at all, leaving our world forever dominated by microbes, never
to welcome sophisticated and amazing life like trees, mushrooms,
caterpillars, and us.
In
1905, the Russian biologist Konstantin Mereschkowski first suggested
that some parts of eukaryotic cells were once endosymbionts—free-living
microbes that took up permanent residence within other cells. He thought
the nucleus originated in this way, as did the chloroplasts that allow
plant cells to harness sunlight. He missed the mitochondria, but the
American anatomist Ivan Wallin pegged them for endosymbionts in 1923.
These ideas were ignored for decades until an American biologist—the late Lynn Margulis—revived them in 1967. In a radical paper,
she made the case that mitochondria and chloroplasts were once
free-living bacteria that had been sequentially ingested by another
ancient microbe. That is why they still have their own tiny genomes and
why they still superficially look like bacteria. Margulis argued that
endosymbiosis was not a crazy, oddball concept—it was one of the most
important leitmotivs in the eukaryotic opera.
The paper was a
tour de force of cell biology, biochemistry, geology, genetics, and
paleontology. Its conclusion was also grossly unorthodox. At the time,
most people believed that mitochondria had simply come from other parts
of the cell. “[Endosymbiosis] was taboo,” says Bill Martin from Heinrich
Heine University Düsseldorf, in Germany. “You had to sneak into a
closet to whisper to yourself about it before coming out again.”
Margulis’
views drew fierce criticism, but she defended with equal vigor. Soon
she had the weight of evidence behind her. Genetic studies, for example,
showed that mitochondrial DNA is similar to that of free-living
bacteria. Now, very few scientists doubt that ancient mergers infused
the cells of every animal and plant with the descendants of bacteria.
“[Endosymbiosis]
was taboo,” says Bill Martin. “You had to sneak into a closet to
whisper to yourself about it before coming out again.”
But
the timing of that merger, the nature of its participants, and its
relevance to the rise of eukaryotes are all still hotly debated. In
recent decades,origin stories for the eukaryotes have sprouted up faster
than old ones could be tested, but most fall into two broad camps.
\
The
first—let’s call it the “gradual-origin” group—claimed that prokaryotes
evolved into eukaryotes by incrementally growing in size and picking up
traits like a nucleus and the ability to swallow other cells. Along the
way, these proto-eukaryotes gained mitochondria, because they would
regularly engulf bacteria. This story is slow, steady, and classically
Darwinian in nature. The acquisition of mitochondria was just another
step in a long, gradual transition. This is what the late Margulis
believed right till the end.
The alternative—let’s call it the
“sudden-origin” camp—is very different. It dispenses with slow,
Darwinian progress and says that eukaryotes were born through the abrupt
and dramatic union of two prokaryotes.
One was a bacterium. The other
was part of the other great lineage of prokaryotes: the archaea. (More
about them later.) These two microbes look superficially alike, but they
are as different in their biochemistry as PCs and Macs are in their
operating systems. By merging, they created, in effect, the starting
point for the first eukaryotes.
Bill Martin and Miklós Müller
put forward one of the earliest versions of this idea in 1998. They
called it the hydrogen hypothesis. It involved an ancient archaeon that,
like many modern members,
drew energy by bonding hydrogen and carbon dioxide to make methane. It
partnered with a bacterium that produced hydrogen and carbon dioxide,
which the archaeon could then use. Over time, they became inseparable,
and the bacterium became a mitochondrion.
There are many
variants of this hypothesis, which differ in the reasons for the merger
and the exact identities of the archaeon and the bacterium that were
involved. But they are all united by one critical feature setting them
apart from the gradual-origin ideas: They all say that the host cell was
still a bona fide prokaryote.
It was an archaeon, through and
through. It had not started to grow in size. It did not have a nucleus.
It was not on the path to becoming a eukaryote; it set off down that
path because it merged with a bacterium. As Martin puts it, “The inventions came later.”
This
distinction could not be more important. According to the sudden-origin
ideas, mitochondria were not just one of many innovations for the early
eukaryotes.
“The acquisition of mitochondria was the origin of
eukaryotes,” says Lane. “They were one and the same event.” If that is
right, the rise of the eukaryotes was a fundamentally different sort of
evolutionary transition than the gradual changes that led to the eye, or
photosynthesis, or the move from sea to land. It was a fluke event of
incredible improbability—one that, as far as we know, only happened
after a billion years of life on Earth and has not been repeated in the 2
billion years since. “It’s a fun and thrilling possibility,” says Lane.
“It may not be true, but it’s beautiful.”
In
1977, microbiologist Carl Woese had the bright idea of comparing
different organisms by sequencing their genes. This is an everyday part
of modern biology, but at the time, scientists relied on physical traits
to deduce the evolutionary relationships between different species.
Comparing genes was bold and new, and it would play a critical role in
showing how complicated life like us—the eukaryotes—came to be.
Woese focused on 16S rRNA,
a gene that is involved in the essential task of making proteins and is
found in all living things. Woese reasoned that as organisms diverge
into new species, their versions of rRNA should become
increasingly dissimilar. By comparing the gene across a range of
prokaryotes and eukaryotes, the branches of the tree of life should
reveal themselves.
They did, but no one expected the results.
Woese’s tree had three main branches. Bacteria and eukaryotes sat on two
of them. But the third consisted of an obscure bunch of prokaryotes
that had been found in hot, inhospitable environments. Woese called them
archaea, from the Greek word for ancient.
Everyone had taken them for
obscure types of bacteria, but Woese’s tree announced them as a third
domain of life. It was as if everyone was staring at a world map, and
Woese had politely shown that a full third of it had been folded
underneath.
These two microbes look
superficially alike, but they are as different in their biochemistry as
PCs and Macs are in their operating systems.
In
Woese’s classic three-domain tree, the eukaryotes and archaea are sister
groups. They both evolved from a shared ancestor that split off from
the bacteria very early in the history of life on Earth. But this tidy
picture started to unravel in the 1990s, as the era of modern genetics
kicked into high gear and scientists started sequencing more eukaryotic
genes.
Some were indeed closely related to archaeal genes, but others
turned out to be more closely related to bacterial ones. The eukaryotes
turned out to be a confusing hodgepodge, and their evolutionary
affinities kept on shifting with every new sequenced gene.
In
2004, James Lake changed the rules of engagement. Rather than looking at
any single gene, he and his colleague Maria Rivera compared the entire
genomes of two eukaryotes, three bacteria, and three archaea. Their
analysis supported the merger-first ideas: They concluded that the
common ancestor of all life diverged into bacteria and archaea, which
evolved independently until two of their members suddenly merged. This
created the first eukaryotes and closed what now appeared to be a “ring
of life.” Before that fateful encounter, life had just two major
domains. Afterward, it had three.
Rivera and Lake were later
criticized for only looking at seven species, but no one could possibly
accuse Irish evolutionary biologist James McInerney of the same fault.
In 2007, he crafted a super-tree using more than 5,700 genes from across
the genomes of 168 prokaryotes and 17 eukaryotes. His conclusion was
the same: Eukaryotes are merger organisms, formed through an ancient
symbiosis between a bacterium and an archaeon.
The genes from
these partners have not integrated seamlessly. They behave like
immigrants in New York’s Asian and Latino communities, who share the
same city but dominate different areas. For example, they mostly
interact with their own kind: archaeal genes with other archaeal genes,
and bacterial genes with bacterial genes. “You’ve got two groups in the
playground and they’re playing with each other differently, because
they’ve spent different amounts of time with each other,” says
McInerney.
They also do different jobs. The archaeal genes are
more likely to be involved in copying and making use of DNA. The
bacterial genes are more involved in breaking down food, making
nutrients, and the other day-to-day aspects of being a microbe. And
although the archaeal genes are outnumbered by their bacterial neighbors
by 4 to 1, they seem to be more important.
They are nearly twice as
active. They produce proteins that play more central roles in their
respective cells. They are more likely to kill their host if they are
mistakenly deleted. Over the last four years, McInerney has found this
same pattern again and again, in yeast, in humans, in dozens of other
eukaryotes.
This all makes sense if you believe the
sudden-origin idea. When those ancient partners merged, the immigrant
bacterial genes had to be integrated around a native archaeal network,
which had already been evolving together for countless generations. They
did integrate, and while many of the archaeal genes were displaced, an
elite set could not be ousted. Despite 2 billion years of evolution,
this core network remains, and retains a pivotal role out of all
proportion to their small number.
The
sudden-origin hypothesis makes one critical prediction: All eukaryotes
must have mitochondria. Any exceptions would be fatal, and in the 1980s,
it started to look like there were exceptions aplenty.
If you
drink the wrong glass of water in the wrong part of the world, your
intestines might become home to a gut parasite called Giardia. In the weeks that follow, you can look forward to intense stomach cramps and violent diarrhea. Agony aside, Giardia
has a bizarre and interesting anatomy. Itconsists of a single cell that
looks like a malevolent teardrop with four tail-like filaments. Inside,
it has not one nucleus but two. It is clearly a eukaryote.
But it has no mitochondria. Mitochondria
(left) are domesticated versions of bacteria (right) that now provide
the cells of every animal, plant and fungus with energy.ShutterstockThere
are at least a thousand other single-celled eukaryotes, mostly
parasites, which also lack mitochondria. They were once called
archezoans, and their missing power plants made them focal points for
the debate around eukaryotic origins. They seemed to be living remnants
of a time when prokaryotes had already turned into primitive eukaryotes,
but before they picked up their mitochondria. Their very existence
testified that mitochondria were a late acquisition in the rise of
eukaryotes, and threatened to deal a knockout blow to the sudden-origin
tales.
That blow was deflected in the 1990s, when scientists slowly realised that Giardia
and its ilk have genes that are only ever found in the mitochondria of
other eukaryotes. These archezoans must have once had mitochondria,
which were later lost or transformed into other cellular compartments.
They aren’t primitive eukaryotes from a time before the mitochondrial
merger—they are advanced eukaryotes that have degenerated, just
as tapeworms and other parasites often lose complex organs they no
longer need after they adopt a parasitic way of life. “We’ve yet to find
a single primitive, mitochondria-free eukaryote,” says McInerney, “and
we’ve done a lot of looking.”
With the archezoan club dismantled,
the sudden-origin ideas returned to the fore with renewed vigor. “We
predicted that all eukaryotes had a mitochondrion,” says Martin.
“Everyone was laughing at the time, but it’s now textbook knowledge. I
claim victory. Nobody’s giving it to me—except the textbooks.”
If mitochondria were so important, why have they only evolved once? And for that matter, why have eukaryotes only evolved once?
Nick Lane and Bill Martin answered both questions in 2010, in a bravura paper called, “The energetics of genome complexity,” published in Nature.
In a string of simple calculations and elegant logic, they reasoned
that prokaryotes have stayed simple because they cannot afford the
gas-guzzling lifestyle that all eukaryotes lead. In the paraphrased
words of Scotty: They cannae do it, captain, they just don’t have the
power.
Lane and Martin argued that for a cell to become more
complex, it needs a bigger genome. Today, for example, the average
eukaryotic genome is around 200,000 times bigger than the average
prokaryotic one. But big genomes don’t come for free. A cell needs
energy to copy its DNA and to use the information encoded by its genes
to make proteins. The latter, in particular, is the most expensive task
that a cell performs, soaking up three-quarters of its total energy
supply. If a bacterium or archaeon was to expand its genome by 10 times,
it would need roughly 10 times more energy to fund the construction of
its extra proteins.
It was as if
everyone was staring at a world map, and Woese had politely shown that a
full third of it had been folded underneath.
One
solution might be to get bigger. The energy-producing reactions that
drive prokaryotes take place across their membranes, so a bigger cell
with a larger membrane would have a bigger energy supply. But bigger
cells also need to make more proteins, so they would burn more
energy than they gained. If a prokaryote scaled up to the same size and
genome of a eukaryotic cell, it would end up with 230,000 times less
energy to spend on each gene! Even if this woefully inefficient wretch
could survive in isolation, it would be easily outcompeted by other
prokaryotes.
Prokaryotes are stuck in an energetic canyon that
keeps them simple and small. They have no way of climbing out. If
anything, evolution drives them in the opposite direction, mercilessly
pruning their genomes into a ring of densely packed and overlapping
genes. Only once did a prokaryote escape from the canyon, through a
singular and improbable trick—it acquired mitochondria.
Mitochondria
have an inner membrane that folds in on itself like heavily ruched
fabric. They offer their host cells a huge surface area for
energy-producing chemical reactions. But these reactions are volatile,
fickle things. They involve a chain of proteins in the mitochondrial
membranes that release energy by stripping electrons from food
molecules, passing them along to one another, and dumping them onto
oxygen. This produces high electric voltages and unstable molecules. If
anything goes wrong, the cell can easily die.
But mitochondria
also have a tiny stock of DNA that encodes about a dozen of the proteins
that take part in these electron-transfer chains. They can quickly make
more or less of any of the participating proteins, to keep the voltages
across their membranes under check. They supply both power and the
ability to control that power. And they do that without having to bother
the nucleus. They are specialized to harness energy. Mitochondria are
truly the powerhouse of the eukaryotic cell. “The command center is too
bureaucratic and far away to do anything,” says Lane. “You need to have
these small teams, which have limited powers but can use them at their
discretion to respond to local situations. If they’re not there,
everything dies.”
If intelligent aliens did exist, they would probably have something like mitochondria, too.
Prokaryotes do not have powerhouses; they are
powerhouses. They can fold their membranes inwards to gain extra space
for producing energy, and many do. But they do not have the secondary
DNA outposts that produce high-energy molecules so the central
government (the nucleus) has the time and energy to undertake
evolutionary experiments.
The only way to do that is to merge
with another cell. When one archaeon did so, it instantly leapt out of
its energetic canyon, powered by its new bacterial partner. It could
afford to expand its genome, to experiment with new types of genes and
proteins, to get bigger, and to evolve down new and innovative routes.
It could form a nucleus to contain its genetic material, and absorb
other microbes to use as new tiny organs, like the chloroplasts that
perform photosynthesis in plants. “You need a mitochondrial level of
power to finance those evolutionary adventures,” says Martin. “They
don’t come for free.”
Lane and Martin’s argument is a huge boon
for the sudden-origin hypothesis. To become complex, cells need the
stable, distributed energy supply that only mitochondria can provide.
Without these internal power stations, other prokaryotes, for all their
evolutionary ingenuity, have always stayed as single, simple cells.
The
kind of merger that creates mitochondria seems to be a ludicrously
unlikely event. Prokaryotes have only managed it once in more than 3
billion years, despite coming into contact with each other all the time.
“There must have been thousands or millions of these cases over
evolutionary time, but they’ve got to find a way of getting along, of
reconciling and co-adapting to each other,” says Lane. “That seems to be
genuinely difficult.”
This improbability has implications for
the search for alien life. On other worlds with the right chemical
conditions, Lane believes that life would be sure to emerge. But without
a fateful merger, it would be forever microbial. Perhaps this is the
answer to the Fermi paradox—the puzzling contradiction between the high apparent odds that intelligent life would exist elsewhere among the billions of planets in the Milky Way,
and our inability to find any signs of such intelligence. As Lane wrote
in 2010, “The unavoidable conclusion is that the universe should be
full of bacteria, but more complex life will be rare.” And if
intelligent aliens did exist, they would probably have something like
mitochondria, too.
The
origin of eukaryotes is, by no means, a settled matter of fact. Ideas
have waxed and waned in influence, and although many lines of evidence
currently point to a sudden origin, there is still plenty of dissent.
Some scientists support radical notions like the idea that prokaryotes
are versions of eukaryotes that evolved to greater simplicity, rather
than their ancestors. Others remain stalwart devotees of Woese’s tree.
Writing
in 2007, Anthony Poole and David Penny accused the sudden-origin camp
of pushing “mechanisms founded in unfettered imagination.” They pointed
out that archaea and bacteria do not engulf one another—that’s a
hallmark of eukaryotes. It is easy to see how a primitive eukaryote
might have gained mitochondria by engulfing a bacterium, but very hard
to picture how a relatively simple archaeon did so.
This
powerful retort has lost some of its sting thanks to a white insect
called the citrus mealybug. Its cells contain a bacterium called Tremblaya, and Tremblaya contains another bacterium called Moranella. Here is a prokaryote that somehow has another prokaryote living inside it, despite its apparent inability to engulf anything.
Still,
the details of how the initial archaeon-bacterium merger happened are
still a mystery. How did one get inside the other? What sealed their
partnership—was it hydrogen, as Martin and Müller suggested, or
something else? How did they manage to stay conjoined? “I think we have
the roadmap right, but we don’t have all the white lines and the
signposts in place,” says Martin. “We have the big picture but not all
the details.”
Perhaps we will never know for sure. The origin of
eukaryotes happened so far back in time that it’s a wonder we have even
an inkling of what happened. Dissent is inevitable; uncertainty,
guaranteed.
“You can’t convince everyone about anything in early
evolution, because they hold to their own beliefs,” says Martin. “But
I’m not worried about trying to convince anyone. I’ve solved these
problems to my own satisfaction and it all looks pretty consistent. I’m
happy.”
