
Why complex life probably evolved only once - hartror
http://www.newscientist.com/article/dn18734-why-complex-life-probably-evolved-only-once.html
======
GavinB
Image that there are several different ways to meet these energy requirements,
and mitochondria just happened to occur first. Mitochondrial life was first
and so it had a chance to evolve and optimized itself. Any new systems that
arose through random mutation would not have had a chance to evolve into a
high-efficiency configuration and would not be able to compete.

Other potential systems may exist and simply never got the foothold necessary
to evolve into an efficient competitor. I would guess that it's quite likely
we're at a local maximum, not the single and only possible way to organize
cellular life.

~~~
jimbokun
"Imagine that there are several different ways to meet these energy
requirements"

OK, what are they?

~~~
farnsworth
"Imagine..."

It isn't necessary to know what they are. GavinB is just saying that there
could be others.

~~~
shadowfox
That comment would be more worthwhile if there were some examples though

~~~
InclinedPlane
He's explaining why we wouldn't see examples of such, even if they did exist.
Whether they could exist or not is another point.

------
jacquesm
One of the best reasons I ever read about why it happened only once is that if
it had happened twice the newcomer would have been called 'food' at a stage
too early to get much further up the ladder.

For it to happen twice the 'new' trail would have to be totally non-nutritious
to the established kind.

I wished I remembered where I read it.

~~~
zeteo
"if it had happened twice the newcomer would have been called 'food' at a
stage too early to get much further up the ladder"

I'm sorry, but this doesn't make any sense. Just being called "food" is not a
sufficient reason for extinction. In fact, organisms at the bottom of the food
chain constitute the majority of biomass in all known ecosystems.

~~~
jacquesm
This talks specifically about those higher up in the food chain, that's what
the 'complex' is all about.

~~~
NickPollard
No, by 'complex' it's talking about non-single-celled organisms. The article
specifically mentions all animals, plants and fungi. In Biological terms,
grass is a complex organism.

~~~
jacquesm
So, let's take the minimal case of 'complex' a system of two cells vs a system
of only one cell.

Any animal from the 'other' branch that observes this 'complex' life from made
of two cells and another one nearby made of only one would have the choice
between two bites for the price of one and one bite.

So new 'complex' life would not be around for very long because the ohter
branch has presumably already reached a higher level of evolution.

It would have be a _very_ successful mutation to stick around long enough to
have enough offspring to take on higher organisms than itself by having some
survive in spite of being predated on.

The lowest level organisms best defense against extinction is their enormous
numbers or symbiotic relationships by evolving in tandem with higher order
forms, a brand new attempt at complexity would not have that advantage yet.

The same goes for another carbon based evolution scenario, after all, if there
could be two independent complex branches of life it would not be too much of
a stretch to think that over the last billions of years a second life form
would have come in to existence. But it would have found each and every niche
already occupied by 'our' kind of life and likely not survive long, and likely
not leave any trace of its existence.

------
eof
This is crap. It doesn't say anything.

Article tl;dr:

Ignoring the inconceivably remote chance of life forming from non-life, it's
very very rare for simple life to evolve into more complicated life. Also, we
just realized that cells need to get bigger to get more complex, and that
somehow isn't just 'another' mutation.

And then the _title_? There is nothing about how or why complex life only
evolved once. Just that it was unlikely. Chance of life forming from non life
* age of the universe * number of stars * the chance a star has an earth like
planet, is far from zero.

I wouldn't even bother ranting but this was number 3 on the front page.

~~~
pygy_
The original article [1] is titled "The energetics of genome complexity",
which is much more reasonable. It argues that the energy processing of
mitochondria was necessary in order for eukaryotes to expand in complexity.

The article is behind a paywall, so I can't read the details.

Anecdotically, there exist anaerobic eukariot cells that live on
hydrogenosomes [2]. I don't know if they were originally aerobic and lost
their mitochondria, or if they evolved in parallel.

There also exist sea slugs that live in symbiosis with the chloroplasts of the
algae they eat [3], so this kind of symbiosis has happened more than once.

1\.
[http://www.nature.com/nature/journal/v467/n7318/full/nature0...](http://www.nature.com/nature/journal/v467/n7318/full/nature09486.html)

2\. <http://en.wikipedia.org/wiki/Hydrogenosome>

3\. <http://www.plantphysiol.org/cgi/content/full/123/1/29>

~~~
nollidge
One of the perks of working for a school... I'll try to summarize as best I
can:

The main calculation in the Nature paper is the energy budget of a cell. They
give the example of _E. coli_ : "to raise gene number tenfold, _E. coli_ must
also increase its energy budget by close to tenfold; and therein lies the
problem." The reason so much more energy is required is due to protein
synthesis (DNA codes to RNA which codes proteins, if you're unfamiliar with
genetics). So obviously with a larger genome, there's more proteins to
synthesize, and this cannot be accomplished by simply making fewer copies of
the old proteins. If anything, you need _more_ of some of those original
proteins as infrastructure support for this new Genome 2.0.

So (and this is my wording) prokaryotes are at a local maximum for genome and
cell size. Chucking mitochondria into the picture provides the wattage
(literally) for them to reach a much greater genome and cell size, and
therefore organismic complexity.

~~~
anigbrowl
I am unclear about why the acquisition of mitochondria by a cell is considered
to be such an unlikely event, as I lean towards the view that given enough
cells/space/time, unlikely short term possibilities become distinct
probabilities eventually. Also, I can't help wondering if the reason that
simple cells are not observed to absorb free mitochondria is that eukaryotic
cells and the organisms they evolved into have somehow poisoned the well, or
attacked newly-complex competitors so aggressively that they've suppressed the
population of omnivorous simple cells.

But I'm no biologist. Am I missing something obvious, or is there a particular
paper explaining why this mitochondrial upgrade is considered such an unlikely
event?

~~~
nollidge
I would guess if one prokaryote engulfs another, it's highly unlikely that
both of them are going to survive the experience, much less that both will
thrive and evolve a symbiotic relationship, much less that this relationship
will persist through cell divisions into subsequent generations. Both
organisms would have to be uniquely suited to the relationship in the first
place.

------
Vivtek
Mitochondrial symbiosis may only have happened once, but I'm not sure I buy
that it's such a leap - mitochondria aren't the only symbiotic organelles.
There are also chloroplasts. So unless chloroplasts are modified mitochondria,
it seems that the eukaryotic leap happened at least twice. That tells me it's
not all _that_ improbable, or so I imagine.

~~~
acqq
Exactly, the existence of chloroplasts (in the language of the the article:
the "power generators" in every plant cell) is an obvious proof that "complex
life probably evolved only once" can't be worse title.

------
hartror
Still a sample size of one, my the drake equation is still looking rather full
of letters. Though it is looking like we may be able to make a decent crack at
fp (fraction of stars that have planets) soon given the success of keplar!

~~~
nooneelse
In a TED talk, Dimitar Sasselov, a co-investigator on the Kepler mission, said
their current best guess for the number of Earth-ish, habitable, planets in
the galaxy is looking to be like 100 million (at about the t=9:20 mark in the
talk). That is, naturally, an early guess from extrapolations and such, but
still it is more based on something than the based-on-nothing and intuition
guesses that sometimes float around in these discussions.

------
billswift
If you are really interested in this, read "Rare Earth",
[http://www.amazon.com/Rare-Earth-Complex-Uncommon-
Universe/d...](http://www.amazon.com/Rare-Earth-Complex-Uncommon-
Universe/dp/0387952896/ref=sr_1_1?ie=UTF8&qid=1287666028&sr=8-1), it goes
through an incredible mass of astronomical, geological, and biological
information. It concludes that, because so many things had to go just right,
and so many others were completely random, primitive life (bacterial-level) is
likely to be more common than previously thought, while advanced life
(multicellular) is likely to be much less common. Because of the sheer size of
the galaxy, it is probable that other intelligences evolved elsewhere, but
they are probably going to be too far away for any sort of contact, even
receipt of potentially meaningful radio. It is a well-written book, despite
the amount of information it is very readable.

------
erikstarck
The number of bacteria on earth is estimated to be
5000000000000000000000000000000. This is five million trillion trillion.

All of them are eating. Some of them are mutating.

Let them do this for a few billions of years and what seems to be a very
unlikely event suddenly becomes almost certain to occur. Complex life FTW!

~~~
Dove
You've got what there, 10^16? That is certainly a very big number. But then,
the article is claiming that the chance of becoming complex is a very small
number. Is it less than 10^-16? (Is it even _knowable_? At a minimum, the
author thinks he can say something about it.) You'd be surprised how fast
these numbers get small if you need to brute force yourself away from a local
maximum and over a valley.

I don't care how big your bignum is compared to everyday experience. Bignum *
Smallnum does not automatically equal certainty, impossibility, or
feasibility. It means _do the math_.

------
boredguy8
The two key arguments in the paper are: 1) Cell membrane invagination isn't
sufficient to overcome the energy well of prokaryotic cells; 2) Endosymbiosis
is the only mechanism to provide the energy necessary to give cells a chance
at complexity.
[http://www.nature.com/nature/journal/v467/n7318/full/nature0...](http://www.nature.com/nature/journal/v467/n7318/full/nature09486.html)

In re: 1,

"Mitochondria must respond quickly to changes in membrane potential and the
penalty for any failure to do so is serious. The electron and proton transfers
of chemiosmotic energy coupling generate a transmembrane potential of 150–200
mV over the membrane (~5 nm across), giving a field strength of about 30
million volt per metre, equal to that discharged by a bolt of lightning. This
high membrane potential sets the inner membrane of bioenergetic organelles
(mitochondria and chloroplasts) apart from all other eukaryotic membrane
systems. Failure to maintain the mitochondrial membrane potential is penalized
by a collapse in energy charge, blocking active transport across the cell
membrane, and a rise in free-radical leak, which in eukaryotes and many
prokaryotes leads directly to programmed cell death.

....

"This requirement for physical association of genes with bioenergetic
membranes to maintain ATP synthesis constrains both the genomes and the
complexity of prokaryotes. If some genes for oxidative phosphorylation must be
physically associated with a certain unit area of bioenergetic membranes, then
beyond that threshold prokaryotes could not maintain membrane potential
homeostasis unless additional genomes are co-localized with the membranes."

In re: 2,

"The main difference between endosymbiosis and polyploidy relates to the size
and distribution of genomes over evolutionary time. In endosymbiosis, surplus
organelle genes are lost or transferred to the host’s chromosomes,
streamlining endosymbiont replication via cytoplasmic inheritance. The outcome
is a massive reduction in genome size, both in prokaryotic endosymbionts and
organelles, with a reciprocal relocation of genes in low copy number to
nuclear chromosomes in the latter. By contrast, in giant polyploid
prokaryotes, all genomes are essentially the same. Without cytoplasmic
inheritance, no genomic specialization ensues.

"In principle, prokaryotes could control respiration using specialized,
membrane-associated plasmids that emulate organelle genomes in gene content
and function. In practice, such plasmids are not found. Bacteria usually have
small, high-copy-number plasmids that segregate randomly at cell division, or
very few giant plasmids that co-segregate with chromosomes on filaments from
midpoint. For plasmids in a prokaryote to support electron flux as organelle
genomes do, high-copy-number giant plasmids encoding components of the
electron-transport chain would need to associate with the plasma membrane, and
evolve counter to the tendency to segregate with size rather than function.
That no mtDNA-like plasmids are known indicates that high energetic barriers
preclude their evolution: unlike organelles, which pay back energetically from
the start, substantial energetic costs must be paid up front (high copy number
of the correct plasmids, and the machinery to associate them with the membrane
at regular intervals) before any energetic advantage can accrue.

....

"Thus, being large and having masses of DNA is not enough to attain
complexity: cells need to control energy coupling across a wide area of
membranes using small, high copy, bioenergetically specialized genomes like
mtDNA. Segregating the genes relinquished by the endosymbiont (mtDNA) into low
copy number in the host’s chromosomes, specialization of the endosymbiont into
an ATP-generating organelle and increasing organelle copy number provides
sufficient energy per gene to support the evolution, maintenance and
expression of some 10^5 more host genes, affording the cell the chance—but not
the necessity—of becoming complex."

~~~
dnautics
beyond that threshold prokaryotes could not maintain membrane potential
homeostasis unless additional genomes are co-localized with the membranes

<http://en.wikipedia.org/wiki/Plasmids>

------
sovande
This correlate fine with what we observe and with the Fermi paradox. That is,
there are no indications of intelligent life elsewhere in the universe. When
Frank Drake first turned his radio telescope to the sky and started to listen,
he expected the ether to be full of radio signals from other civilizations. To
his astonishment it was silent. Obviously there is a "great filter" in place
which prevent intelligent life to evolve in abundance. And this article seems
to point to such a filter. Nick Bostrom has an interesting philosophical take
on this <http://www.nickbostrom.com/papers/fermi.pdf>

------
VladRussian
there is some fallacy in such discussing of the precise state of chaotic
dynamic systems.

It is not possible to exactly reproduce the state we have with life on Earth,
like it isn't possible to get precisely the same weather in 2 different days -
both are results of unimaginably many values behaving and interacting in
unique way and even minor change will change the resulting outcome . Yet there
are a lot of days that look and feel very similar and there is 99.9999999%
chances that tomorrow, for example, we wouldn't have 150 F in San-Francisco
while we can't be sure what precisely it will be - 60 or 70 or 50?

Does the life evolution system have such bound-ness and attractor-ness
properties - well, as of now we can only guess. My guess is "yes".

Btw. Uniqueness because of complexity, in the sense of "evolved only once",
stinks like Creationism/ID

------
CapitalistCartr
We know the odds of evolution are low; the point is that the number of stars
in the Universe is astoundingly huge. With at least 10^20 stars in the
Universe, those low odds get much more reasonable, especially now it's
starting to look like planets are common.

~~~
Dove
Do you know the actual odds, though? I mean, there's low and then there's
_low_.

If evolution on a habitable planet has a 10^-4 chance of happening, then the
universe is almost certainly teeming with life. If it's 10^-18, the size of
the universe is the difference between believing and doubting the idea. If
it's 10^-200, the universe barely helps at all, and you need to start talking
about billions upon billions of barren universes and we just happen to live in
this one.

Do you know what the number is? Do you know where it lies between 0.9 and
10^-(10^10^10)? Do you know whether the 20 orders of magnitude you get from
the stars is overwhelming, decisive, or not even enough to make a dent?

~~~
hugh3
To my way of thinking, the fact that life showed up on Earth relatively
quickly, within half a billion years of the planet's formation, suggests that
life is relatively easy. The fact that complex life took another few billion
years after that suggests that simple to complex is the difficult step. But
the fact that we nonetheless find ourselves here around a live-fast-die-young
yellow star when the universe is only 14 billion years old rather than in
close orbit around a dim red star a hundred billion years later suggests to me
that it's probably not a one-in-every-zillion universes sort of event.

~~~
petervandijck
That sounds reasonable enough to me.

[big bang --(we're here-
ish)------------------------------------------------------- end of universe]

So the facts that we're that far to the left of that line, and that the
universe(s) is/are really really big, suggest that life isn't that rare.

------
pavel_lishin
So, we go from "Simple cells __hardly ever __engulf other cells" to "it
has/will never happen anywhere in the universe"?

~~~
hugh3
Blame the headline writer, but the actual article is still pretty interesting.

------
dnautics
I'm sorry, but the authors of this paper are totally clueless.

"whenever simple cells start to become more complex"

The definition of complexity here is irrigorous to the point of laughability.
At one point the definition makes a call about "size" - " if a bacterium grew
to the size of a complex cell, it would run out of juice"

Nonsense. There was recently discovered a macroscopic bacterium -
<http://en.wikipedia.org/wiki/Thiomargarita_namibiensis> Yes, you can see them
with your own eyes.

"If the energy-producing machinery straddling the membrane is not constantly
fine-tuned, it produces highly reactive molecules that can destroy cells. Yet
fine-tuning a larger membrane is problematic because detecting and fixing
problems takes longer."

The solution, then is to create short-length response pathways. Presumably the
"reactive molecule" they're talking about is superoxide - Life has evolved an
incredibly awesome enzyme that takes care of it - superoxide dismutase - and
even a slightly defective SOD doesn't lead to inviability. Certain types of
photosynthetic bacteria, which need to "carefully manage reactivity" have
solved this problem by creating 'hydrogenases' which bleed off excess energy
in the form of hydrogen molecules (these may become useful in the alternative
fuel industry).

"Acquiring mitochondria, it seems, was a one-off event."

Well, no. Plants later acquired chloroplasts. Some organisms have
hydrogenosomes (which is inetresting, because we now have discovered
vertebrates that seem to be able to survive off their hydrogenosomes,
mitochondria be damned). Some bacteria have evolved anammoxasomes.

"cells capable of complex functions – such as communicating with each other
and having specialised jobs – could evolve"

Clearly the author has not heard of quorum sensing. The notion that bacteria
are hyperindividualistic robinson crusoes adrift in their environment is
generally not believed anymore. <http://en.wikipedia.org/wiki/Quorum_Sensing>

Then the author redefines complexity to be a 'large genome' thing. Of course
there really isn't anything stopping bacteria from having ploidy. Deincococcus
Radiotolerans, for example, has 4-10 copies of its genome inside itself (for
reasons that you might be able to guess from its name).
<http://en.wikipedia.org/wiki/Deinococcus_Radiotolerans>

And the whole complexity thing is bunk anyways. The actinomyces are such
complex bacteria that for many many years scientists thought they were fungi.
<http://en.wikipedia.org/wiki/Actinomyces>

It's also worth noting, that there certainly are eukarya that are plenty
'complex' that simply don't have mitochondria.
<http://en.wikipedia.org/wiki/Giardia>

Better explanation: Eukarya were enabled because the Great Oxygen Catastrophe
suddenly made being a paired oxidizer (mitochondrion)-reducer (host cell)
beneficial. Prior to the oxygen catastrophe, mitochondrial cells would have
only been able to survive in homeostasis with a cell that was happy to have
its poop (oxygen) taken care of locally... Not to mention that eukarya are
generally considered to have branched off of archae... The specialness of
eukarya is that they were a unification of two highly divergent branches of
the phylogenetic tree.

------
msg
_Or, as Lane puts it: "The underlying principles are universal. Even aliens
need mitochondria."_

Right, midichlorians are in all living things of sufficient complexity.

Episode 1 was onto something?

Kill me quickly.

