When you were a tadpole

1.17-1.10 billion years ago. The atmosphere is one percent oxygen or so thanks to photosynthetic algae. The ocean still largely anoxic and thick with sulfates and sulfate-eating bacteria. Eukaryotes have been around for a while, and are diversified, although still all single celled (as far as we know).

Sexual reproduction begins with eukaryotes, and by now some groups are presumably differentiated into male and female. For those of you who are not bdelloid rotifiers, here’s a poem for Valentine’s Day, by the biologist Langdon Smith. Martin Gardner has a nice account of the poem, in his book “When you were a tadpole and I was a fish,” and here’s a great video of the poem spoken by Jean Shepherd.

Evolution
By Langdon Smith (1858-1908)

When you were a tadpole and I was a fish
In the Paleozoic time,
And side by side on the ebbing tide
We sprawled through the ooze and slime,
Or skittered with many a caudal flip
Through the depths of the Cambrian fen,
My heart was rife with the joy of life,
For I loved you even then.

Mindless we lived and mindless we loved
And mindless at last we died;
And deep in the rift of the Caradoc drift
We slumbered side by side.
The world turned on in the lathe of time,
The hot lands heaved amain,
Till we caught our breath from the womb of death
And crept into life again.

We were amphibians, scaled and tailed,
And drab as a dead man’s hand;
We coiled at ease ‘neath the dripping trees
Or trailed through the mud and sand.
Croaking and blind, with our three-clawed feet
Writing a language dumb,
With never a spark in the empty dark
To hint at a life to come.

Yet happy we lived and happy we loved,
And happy we died once more;
Our forms were rolled in the clinging mold
Of a Neocomian shore.
The eons came and the eons fled
And the sleep that wrapped us fast
Was riven away in a newer day
And the night of death was passed.

Then light and swift through the jungle trees
We swung in our airy flights,
Or breathed in the balms of the fronded palms
In the hush of the moonless nights;
And oh! what beautiful years were there
When our hearts clung each to each;
When life was filled and our senses thrilled
In the first faint dawn of speech.

Thus life by life and love by love
We passed through the cycles strange,
And breath by breath and death by death
We followed the chain of change.
Till there came a time in the law of life
When over the nursing sod
The shadows broke and the soul awoke
In a strange, dim dream of God.

I was thewed like an Auroch bull
And tusked like the great cave bear;
And you, my sweet, from head to feet
Were gowned in your glorious hair.
Deep in the gloom of a fireless cave,
When the night fell o’er the plain
And the moon hung red o’er the river bed
We mumbled the bones of the slain.

I flaked a flint to a cutting edge
And shaped it with brutish craft;
I broke a shank from the woodland lank
And fitted it, head and haft;
Then I hid me close to the reedy tarn,
Where the mammoth came to drink;
Through the brawn and bone I drove the stone
And slew him upon the brink.

Loud I howled through the moonlit wastes,
Loud answered our kith and kin;
From west to east to the crimson feast
The clan came tramping in.
O’er joint and gristle and padded hoof
We fought and clawed and tore,
And cheek by jowl with many a growl
We talked the marvel o’er.

I carved that fight on a reindeer bone
With rude and hairy hand;
I pictured his fall on the cavern wall
That men might understand.
For we lived by blood and the right of might
Ere human laws were drawn,
And the age of sin did not begin
Til our brutal tush was gone.

And that was a million years ago
In a time that no man knows;
Yet here tonight in the mellow light
We sit at Delmonico’s.
Your eyes are deep as the Devon springs,
Your hair is dark as jet,
Your years are few, your life is new,
Your soul untried, and yet –

Our trail is on the Kimmeridge clay
And the scarp of the Purbeck flags;
We have left our bones in the Bagshot stones
And deep in the Coralline crags;
Our love is old, our lives are old,
And death shall come amain;
Should it come today, what man may say
We shall not live again?

God wrought our souls from the Tremadoc beds
And furnish’d them wings to fly;
He sowed our spawn in the world’s dim dawn,
And I know that it shall not die,
Though cities have sprung above the graves
Where the crook-bone men made war
And the ox-wain creaks o’er the buried caves
Where the mummied mammoths are.

Then as we linger at luncheon here
O’er many a dainty dish,
Let us drink anew to the time when you
Were a tadpole and I was a fish.

Between Darwin and Saint Valentine

Yesterday was Darwin’s birthday (and Lincoln’s). Tomorrow is Valentine’s Day. Here’s a post appropriate for either day.

Imagine sex worked like this:

You’ve been feeling bad lately, getting sick a lot. You’re not at your best. You find someone who seems to be in better shape. One thing leads to another and you wind up acquiring body fluids from the other party – and picking up some new genes from them. The new genes help a lot in fighting off infection. You’re feeling better now.

Reproduction? That’s another matter, nothing directly to do with sex. When you reproduce, your offspring will carry all the genes you happen to have at the moment.

Also, I forgot to mention that you’re neither male or female – the gene exchange could have gone in the other direction if you’d both been in the mood. And your partner in the adventure above might not even have been the same species as you.

This is more or less how bacteria work out sex. (Joshua Lederberg got the Nobel Prize for figuring this out.) Eukaryotes (you’re one of them) mostly do it differently, combining sex and reproduction. It’s the story you learned in high school about passing on half your genes to a gamete (sex cell), which joins with another gamete to make a new organism.

Most eukaryotes also have two sexes. The best theory we have about why that got started goes like this: Most of the DNA in a eukaryote cell is in the nucleus. But a small fraction is in the mitochondria, little powerhouses outside the nucleus that started out as bacteria, and got domesticated. Imagine that two gametes join together, and combine two sets of mitochondria. There’s a potential conflict here. Suppose your mitochondria have a mutation that lets them clobber your partner’s mitochondria. This is good (evolutionarily speaking) for the winning mitochondria, but very likely to be bad for the cell as a whole. Better for the cell as a whole is if one gamete, acting on instructions from the nucleus, preemptively clobbers all their own mitochondria, so that all the mitochondria come from just the other gamete. This is the beginning of what will eventually lead to a distinction between sperm and eggs, pollen and ovules, male and female. Which means you got all your mitochondrial DNA from your mom, something that will turn out to be important when we look later in the year at geneticists unraveling human prehistory. This is also an example of how selection at one level (within cells) can conflict with selection at another level (between cells). We’ll see such multilevel selection again and again, for example in the evolution of complex human societies.

Sex has to be highly advantageous, although we’re not sure exactly what the advantage is. When eukaryote species give it up, they don’t seem to last long. Dandelions reproduce asexually: based on what we see in other organisms, they probably won’t be around for long, evolutionarily speaking. There’s one mysterious exception, tiny animals called bdelloid rotifers which have been reproducing asexually for tens of millions of years . For readers who are not bdelloid rotifers: Happy Valentine’s Day tomorrow! We’ll have an appropriate evolutionary post up tomorrow.

Life goes nuclear

2.05-1.93 Bya

Eukaryotic cells (Domain Eucaryota, which includes multicellular life, like plants, animals, and fungi) are, on average, much larger and more complex than the earlier evolved prokaryote cells (Domains Bacteria and Archaea*). They have organelles, including mitochondria that power them and chloroplasts (at least among plants) that carry out photosynthesis. Their DNA is stored in a nucleus, and consists not just of genes (as in prokaryotes), but of large stretches of non-coding DNA (most of their genome), separating pieces of genes. The ancestor of present-day eukaryotes reproduced sexually, although some eukaryotes have since given up sex.

There are different ways that life increases in complexity. The origin of the Eucarya has something in common with a much later event, the origin of agriculture (coming up September 9 on logarithmichistory!). Starting 10,000 years ago, we Homo sapiens brought other animals and plants under our control, managing their reproduction, and selecting them (first unintentionally, then intentionally) to suit our purposes, until now most domesticated creatures couldn’t survive in the wild. Our own numbers and social scale increased enormously with the rise of agriculture.

At least 2 billion years ago, an archaeon cell gobbled up one or more bacterial cells (or was parasitized by them). The bacteria ended up surviving inside it, and after many generations became a kind of domesticate inside their host. Eukaryotes do domestication one better than humans: they carry their livestock inside their bodies. Eventually this domesticate evolved into mitochondria, the little power packs that pump out ATP for the rest of the cell to use as as an energy source. Over the course of time all but a small fraction of the original bacterial genome was moved into the nucleus. Archaean and bacterial genomes are so intertwined, that the evolution of eucaryotes is better represented as a ring than a tree.

ringoflife

Humans developed agriculture multiple times independently around the world. As far as we know, eukaryotes evolved only once. The evolution of eukaryotes might be a very unlikely chance event. The universe may be full of bacteria, but harbor more complex cells only sparsely.

 

* Domain Archaea, a billions-of-years-old phylum of single-celled organisms looking like bacteria but biochemically different, should not be confused with the Archaean Eon, a billions-of-years-long stretch of Earth history.

Slow weather and the world inside

Inside planet Earth is another planet, the size of Mars. Not quite the way Jules Verne or Edgar Rice Burroughs imagined, but …

We live at the boundary between Earth’s solid surface, its liquid water surface, and its atmosphere. The sun pours energy into this boundary, and drives the circulation of water and air. The altitude of the surface varies. Events on this boundary rule our lives. We live at the mercy of weather, worry about climate change and rising sea levels, and (if we are Tibetans or Andean Indians) have genes adapting us to life at high altitude.

There is an equally dramatic landscape 1800 miles deep in the Earth, at the boundary between the oxygen-rich silicate mantle and the iron core. The core-mantle boundary is as dramatic a transition as the Earth-atmosphere boundary. And the boundary is not smooth or featureless: We know, from measuring the choppy irregular pattern of seismic waves that bounce off it, that there is a complex uneven landscape at the boundary. There is a kind of slow weather in the mantle, driven not by the sun, but by heat from the core. The core-mantle boundary has cool uplands where material falling faintly through the viscous mantle from above lays thickly drifted, in mountains hundreds of miles high. It has torrid lowlands, from which hotter, less viscous plumes of magma rise, to generate hotspots far above. (Two of the major plumes nowadays are under the central Pacific and Africa.) And there may also be lakes and seas of liquid silicates at the boundary.

Long-term geological processes – the movement of tectonic plates, the formation and breakup of continents – are driven by this slow weather in the mantle. One theory is that things changed around the end of the Archaean and beginning of the Proterozoic — that the “weather” inside the Earth grew calmer, the lower and upper mantle grew more separate, and the modern pattern of supercontinent formation and breakup began. If this is true, then changes we see at the surface of the Earth, like the rise of an oxygen atmosphere, may owe something to events where mantle meets core.

Better living through chemistry

3.20-3.04 billion years ago

Chemistry plays a big role once Earth forms. Different mineral species appear, with different chemical compositions. Magnesium-heavy olivine sinks to the lower mantle of the Earth. Aluminum-rich feldspars float to the top.

Chemistry is an example of what William Abler calls “the particulate principle of self-diversifying systems,” what you get when a collection of discrete units (atoms) can combine according to definite rules to create larger units (molecules) whose properties aren’t just intermediate between the constituents. Paint is not an example. Red paint plus white paint is just pink paint. But atoms and molecules are: two moles of hydrogen gas plus one mole of oxygen gas, compounded, make something very different, one mole of liquid water.

A lot of important chemical principles are summed up in the periodic table.

periodictable copy

On the far right are atoms that have their electron shells filled, and don’t feel like combining with anyone. Most, but not all the way, to the right are atoms with almost all their shells filled, just looking for an extra electron or two. (Think oxygen, O, with slots for two extra electrons). On the left are atoms with a few extra electrons they can share. (Think hydrogen, H, each atom with an extra electron it’s willing to share with, say, oxygen.) In the middle are atoms that could go either way: polymorphously perverse carbon, C, with four slots to fill and four electrons to share, and metals, that like to pool their electrons in a big cloud, and conduct electricity and heat easily. (Think of Earth’s core of molten iron, Fe, a big electric dynamo.)

Another example of “the particulate principle of self-diversifying systems” is human language. Consider speech sounds, for example. You’ve got small discrete units (phonemes, the sounds we write bpskchsh, and so on) that can combine according to rules to give syllables. Some syllables are possible, according to the rules of English, others not. Star and spikythole and plast, are possible English words, tsar and psyche are not (at least if you pronounce all the consonants, the way Russians or Greeks do), nor tlaps nor bratz (if you actually try to pronounce the z). Thirty years ago appblog, and twerk were not words in the English language, but they were possible words, according to English sound laws.

You can make a periodic table of consonants.

phonemes

Across the top are the different places in the vocal tract where you block the flow of air. Along the left side are different ways of blocking the flow (stopping it completely –t-, letting it leak out –s-, etc.) The table can explain why, for example, we use in for intangible and indelicate, but switch to im for impossible and imbalance. (The table contains sounds we don’t use in English, and uses a special set of signs, the International Phonetic Alphabet, which assigns one letter per phoneme.) This is why a book title like The Atoms of Language makes sense (a good book by the way).

So sometimes the universe gets more complex because already existing stuff organizes itself into complex new patterns  – clumps and swirls and stripes. But sometimes the universe gets more complex because brand new kinds of stuff appear, because a new particulate system comes online: elementary particles combine to make atoms, atoms combine to make molecules, or one set of systems (nucleotides to make genes, amino acids to make proteins) combines to make life, or another set of systems (phonemes to make words, words to make phrases and sentences) combines to make language.

My name is LUCA. I live on the ocean floor.

How life began on Earth is still not well understood. The “RNA world” is one popular theory. In modern organisms, nucleic acids, DNA and RNA, store and transfer information, but proteins do the actual work of catalyzing chemical reactions. But RNA can act as a catalyst, so maybe the first replicating systems involved RNA catalyzing its own replication. However, RNA doesn’t spontaneously form very easily, so it’s not clear how the RNA world would have gotten started. Borate minerals might help but it’s not clear they were around that early.

Another possibility that’s gotten some attention lately involves droplets that grow and divide, instead of just merging into bigger drops.

A different approach to the topic is to work backward from living organisms, to reconstruct the biochemistry of LUCA, the Last Universal Common Ancestor (not quite the same as the first living thing). Recent research on these lines implies that LUCA was a heat-loving microbe that relied on hydrogen as its energy source, suggesting an undersea volcano as a habitat.

However the first organisms got established on Earth, it happened very quickly. Just about as soon as the planet could support life we find chemical evidence for it, from Isua, Greenland (but no fossils yet). This suggests that the origin of life is pretty easy (unless we want to go with panspermia). Mars may have been a more habitable place early in its history, and perhaps Mars exploration will one day solve the mystery of the origin of life in our Solar System.

Sun, Earth, Moon, Earthrise

4.56 billion years ago

The Hadean eon begins with the origin of the Earth 4.56 Bya.

Take a look at the Moon tonight. It’s waning, just past half full. When the moon is full you can cover it with your thumb. 4.56 billion years ago the new moon was ruddy with volcanic activity even on its dark side. The Moon seen from Earth was 16 times wider, covering 250 times more sky, and 250 times brighter when full. That is what you would have seen just after Earth acquired a surface you could stand on, although you would have needed an oxygen mask. And watch out for massive meteorites, still falling frequently, and volcanism.

Chance events late in the history of planet formation played a huge role in shaping the solar system, including the collision with the planet Theia (named after the Greek goddess of the Moon) that gave Earth her outsized satellite. Life might have developed very differently – there might be no intelligent life — without the Moon’s influence on tides and on Earth’s axis.

Here’s what Earth and Moon look like today: the famous picture of Earthrise, taken December 24, 1968, by William Anders abroad Apollo 8.earthrise copy

In 1969, the Brazilian singer Caetano Veloso was imprisoned by Brazil’s military dictatorship. He was expelled from the country and lived in exile until 1972. In prison he saw a picture of the Earth from space and wrote this song, “Terra” (Earth).

Terra

Quando eu me encontrava preso, na cela de uma cadeia 
Foi que eu vi pela primeira vez, as tais fotografias 
Em que apareces inteira, porém lá não estava nua 
E sim coberta de nuvens
Terra, terra, Por mais distante o errante navegante Quem jamais te esqueceria?

Ninguém supõe a morena, dentro da estrela azulada
. Na vertigem do cinema, mando um abraço pra ti 
Pequenina como se eu fosse o saudoso poeta 
E fosses a Paraíba
Terra, terra, 
Por mais distânte o errante navegante Quem jamais te esqueceria

Eu estou apaixonado, por uma menina terra,
 Signo de elemento terra. Do mar se diz terra à vista 
Terra para o pé firmeza, terra para a mão carícia
 Outros astros lhe são guia
Terra, terra,
 Por mais distânte o errante navegante Quem jamais te esqueceria

Eu sou um leão de fogo, sem ti me consumiria
 A mim mesmo eternamente, e de nada valeria 
Acontecer de eu ser gente. e gente é outra alegria 
Diferente das estrelas
Terra, terra,
 Por mais distânte o errante navegante Quem jamais te esqueceria

De onde nem tempo e nem espaço, que a força te de coragem
 Pra gente te dar carinho, durante toda a viagem 
Que realizas do nada, através do qual carregas 
O nome da tua carne
Terra, terra, 
Por mais distânte o errante navegante Quem jamais te esqueceria
Terra, terra, 
Por mais distânte o errante navegante Quem jamais te esqueceria
Terra, terra,
 Por mais distânte o errante navegante Quem jamais te esqueceria?

Na sacadas do sobrado, Da eterna São Salvador 
Há lembranças de donzelas, do tempo do Imperador
 Tudo, tudo na Bahia faz a gente querer bem
A Bahia tem um jeito
Terra, terra,
 Por mais distante o errante navegante 
Quem jamais te esqueceria. Terra

Earth

When I found myself arrested
 In a prison cell
, That’s when I first saw
 Those famous pictures
 In which you appear entire, 
However you were not naked 
But covered by clouds.
Earth! Earth!
 However distant 
The wandering navigator 
Who could ever forget you?

Nobody thinks of the brunette
 Inside the bluish star. 
In the vertigo of the movie
 I send you an embrace, 
Little one – as if I were
 the homesick poet 
And you were the Paraíba
Earth! Earth!
 However distant 
The wandering navigator 
Who could ever forget you?

I’m just in love 
With an earth girl, 
Sign of the element “Earth.” 
From the sea is said “Land in sight.” 
Earth to the foot: solidity. 
Earth to the hand: a caress.
 Other stars are guides for you
Earth! Earth! 
However distant 
The wandering navigator 
Who could ever forget you?

I am a lion of fire
 Without you 
I would burn myself up eternally 
And it would be worth nothing, 
The fact of my being human. 
And human is another joy 
Different than the stars’
Earth! Earth! 
However distant 
The wandering navigator 
Who could ever forget you?

From where there’s neither time nor space 
May the force send courage 
For us to treat you tenderly
 During all the journey 
That you carry out through nothing
 Through which you bear
 The name of your flesh
Earth! Earth!
 However distant 
The wandering navigator 
Who could ever forget you?
Earth! Earth! 
However distant
 The wandering navigator
 Who could ever forget you?
Earth! Earth! 
However distant
 The wandering navigator
Who could ever forget you?

In the townhouses’ terraces 
Of eternal Salvador 
There are reminders of maidens 
From the time of the Emperor 
Everything, everything in Bahia
 Makes us fond 
Bahia has such a way.
Earth! Earth! 
However distant
 The wandering navigator
 Who could ever forget you? 
Earth