Notes From an Apocalypse

(This is a loose adap­ta­tion of a talk I some­times give on the Cam­brian Ex­plo­sion, smoothed a bit for pop­u­lar con­sump­tion. That talk, in turn, draws heav­ily from a 2006 pa­per by Charles Mar­shall, ti­tled “Ex­plain­ing the Cam­brian ‘Ex­plo­sion’ of An­i­mals”. It can be read in full here. I’m mostly just try­ing to test out the new site with an es­say I had ly­ing around, be mod­er­ately en­ter­tain­ing, and maybe try to sug­gest this event as a topic of in­ter­est for those who care about in­tel­li­gence ex­plo­sions and cog­ni­tive emer­gence. If I suc­ceed in all three, then I en­courage you to start with Mar­shall for the more tech­ni­cal, thor­ough, and cor­rect anal­y­sis.)

A.

The Cam­brian Ex­plo­sion is our name for an event that took place about 540 mil­lion years ago, one that ac­counts for the sud­den ap­pear­ance of ad­vanced an­i­mal life. Em­pha­sis on “sud­den”; it’s like some­body flipped a light switch. What fol­lows is just a story (frag­ments of a story, re­ally), but it’s a pretty good one, and I don’t think it’ll lead you too far astray.

Let’s start 540 mil­lion years ago. The planet is still re­cov­er­ing from a se­ries of catas­trophic “snow­ball Earth” phases, ice ages so se­vere that the oceans had frozen right across the equa­tor; the trop­i­cal glaciers wound their way through loose sed­i­ment to leave an en­dur­ing foot­print. Now that the last of these has thawed, it’s fi­nally start­ing to warm up in a more per­ma­nent way and get a lit­tle more hos­pitable. There are two ma­jor con­ti­nents at the mo­ment, but one of them is start­ing to break up into smaller and smaller chunks. As with our ris­ing tem­per­a­tures, this helps cre­ate a more hab­it­able planet, since it gets you more high-nu­tri­ent coastlines per unit land area. Im­por­tantly, the last hun­dred mil­lion years have pro­vided us with oxy­gen con­cen­tra­tions in the at­mo­sphere at ba­si­cally 21st-cen­tury lev­els. To the un­pre­pared or­ganisms that dom­i­nate the first half of Earth’s his­tory, oxy­gen can act as a haz­ardous toxin, but if you har­ness it cor­rectly you can re­ally kick your metabolism into a higher gear. A good time to be al­ive, re­ally.

The land is mostly bar­ren. A lit­tle bit of pond scum hold­ing on in shal­low pools and other moist ar­eas, maybe, but seeds and pla­cen­tas won’t ex­ist for a long age yet, and so far ev­ery re­pro­duc­tive sys­tem re­quires the pres­ence of stand­ing wa­ter. The ocean, though, the ocean is a differ­ent story, full of life and liv­ing. If you go by the rock record, the most com­mon form of life is the stro­ma­to­lite, a colony of green plant-microbes that se­crete stone in dis­tinc­tive whor­ling pat­terns. (As always, his­tory most re­mem­bers those who write his­tory). They flour­ish in shal­low and sun­light-touched wa­ters, grad­u­ally con­struct­ing ranges of gen­tly rol­ling hills that are each some­times me­ters across. Larger (but still micro­scopic) Eukary­otes wan­der through these hills like graz­ing deer. Or maybe like wolves. Un­like the helpful stro­ma­to­lites them­selves, most of these crea­tures rarely leave in­for­ma­tive fos­sils, so it’s hard to say how com­pli­cated the ecosys­tem ac­tu­ally is. Cer­tainly it’s a rich and dy­namic biolog­i­cal scene, but to my mind it always feels just a lit­tle bit like Hob­biton. Gen­tle with a hid­den strength, sim­ple enough and flex­ible enough to en­dure. The offi­cial name for this time pe­riod is the Protero­zoic, but when ge­ol­o­gists think no­body is listen­ing, we some­times call it the Bor­ing Billion.

But here in the clos­ing years of that era, as tem­per­a­tures thaw and the coastlines un­wind and the at­mo­sphere fills with oxy­gen, it’s start­ing to get a bit more in­ter­est­ing. Big. Some of the Eukary­otes are be­hav­ing oddly, bunch­ing up into colonies that are a bit bet­ter at find­ing nu­tri­ents or sun­light. Often, they give up roam­ing and gather to­gether in big fronds, an­chor­ing them­selves to the seafloor and spread­ing the sail out broad­side to catch as much nu­tri­ent-rich ocean wa­ter as pos­si­ble as it flows by. Sporifera, the sponges, use a differ­ent strat­egy, puls­ing flag­ella in­wards to cre­ate lit­tle ar­tifi­cial cur­rents that chan­nel nu­tri­ents to­wards them­selves. Cnidaria, the jel­lyfish, ac­tu­ally lift off from the ocean floor al­to­gether and drift through the wa­ter look­ing for scraps among the drift­ing plank­ton. The cells in these colonies are start­ing to take on spe­cial­ized roles, this one bind­ing the group to the soil, that one a ten­ta­cle, but their over­all sim­plic­ity falls well short of what we usu­ally mean by words like ‘an­i­mal.’

De­spite these omens, it’s all rather se­date. Not much seems to change day to day, mil­le­nium to mil­le­nium. You’d be for­given for check­ing your watch a few times ev­ery epoch. You might even take a nap, stay in bed for a few mil­lion years and re­lax. Maybe twenty mil­lion if you hit the snooze but­ton. But when you wake up, you’ll be in for a shock.

B.

Some­body broke it. They scourged the Shire. In just a few mil­lion years, no more than a wink of ge­olog­i­cal time, ev­ery­thing has changed. Many of those ever-green stro­ma­to­lite hills have been stripped bare, and the ones that re­main are go­ing fast. Gi­ant mon­sters prowl through the wreck­age, hunt­ing for any­thing made out of com­plex or­gan­ics, hunt­ing each other. There is a be­wil­der­ing ar­ray of new life­forms. Some of them are cov­ered in spikes, oth­ers with odd num­bers of eyes and strange prob­ing ten­ta­cle-mouths. All of them have elab­o­rate or­gan sys­tems, strange tis­sue masses that ex­press them­selves in rad­i­cally differ­ent, in­ter­lock­ing ways, de­spite hav­ing the same genome. The rise in di­ver­sity, and in dis­par­ity, is un­equaled by any other mo­ment in Earth’s his­tory. Some­thing like half of all 21st cen­tury an­i­mal phyla trace their ori­gins back to that brief mo­ment of gen­er­a­tion. Take all the cre­ative power of the last five hun­dred mil­lion years of an­i­mal evolu­tion, com­press it down to a frac­tion of a ge­olog­i­cal in­stant- that’s the power of the Cam­brian Ex­plo­sion. In less than twenty mil­lion years, there are mol­luscs squirm­ing like mod­ern sea urch­ins, ech­in­o­derms cling­ing to rocks like mod­ern starfish. Even the trilo­bite, that an­cient sym­bol of an­cient life, sud­denly ap­pears here fully-formed. And then, swim­ming through the open wa­ters, you’d see the most sur­pris­ing thing of all: one of them has a brain. An­i­malia Bila­te­ria Chor­data, the chor­date.

This is it, you see. The mo­ment of en­cephal­iza­tion, the mo­ment that the cos­mos wakes up.

There was the Earth, a gi­ant rock drift­ing quietly through space. And one day it just spon­ta­neously grows a brain, for no damn rea­son at all. What kind of rock does that, ex­actly? What kind of uni­verse?

C.

No, re­ally, why did this hap­pen? What was the mechanism, the bridge that takes us from the bor­ing billion to the era of minds and mon­sters?

I don’t know.

Really, I don’t. There are hy­pothe­ses, sure. A few scraps of al­most-un­der­stand­ing. And plenty of guesses, some of them re­ally good. But the stone can only tell us so much, and it was such a very long time ago. The all-im­por­tant mid­dle of our story, the one that gives us the first mo­ments of emerg­ing biolog­i­cal con­scious­ness, has not yet been re­cov­ered.

Can it be re­cov­ered, even in prin­ci­ple? Harder ques­tion.

Dar­win, poor Charles Dar­win, floundered on these shoals with the rest of us. For him, the only ex­pla­na­tion was that we must be miss­ing some huge frac­tion of the rock, so that it only seems like they all show up at once:

I can­not doubt that all the Silurian trilo­bites have de­scended from some one crus­tacean, which must have lived long be­fore the Silurian age....Con­se­quently, if my the­ory be true, it is in­dis­putable that be­fore the low­est Silurian strata was de­posited, long pe­ri­ods elapsed, as long as, or prob­a­bly longer than, the whole in­ter­val from the Silurian to the pre­sent day.....The case must at pre­sent re­main in­ex­pli­ca­ble; and may be tru­ely urged as a valid ar­gu­ment against the views here en­ter­tained.

He was wrong. Iso­topic dat­ing meth­ods have since con­firmed that there is no gap in the rock be­tween the Protero­zoic and the Cam­brian, no space of hun­dreds of mil­lions of years for us to move grad­u­ally from one ex­treme to the other. (In this case, we’re also early enough in the his­tory of ge­ol­ogy that pre­cise chronol­ogy was still am­bigu­ous- that’s why Charles refers here to the Silurian, a later era.)

And so, as the man says, the Cam­brian Ex­plo­sion may be truly urged as a valid ar­gu­ment against the views which Mr. Dar­win en­ter­tained. What’s at stake here is not just the ques­tion of how this pro­cess of en­cephal­iza­tion first oc­curred, but also why our most foun­da­tional biolog­i­cal the­o­ries fail so spec­tac­u­larly to an­ti­ci­pate it. Are we even ask­ing the right ques­tions?

D.

There’s a slightly more mod­ern ver­sion of Dar­win’s first ten­ta­tive at­tempts at a patch, one that might ex­plain why so many fos­sils would be miss­ing from the pre-Cam­brian record. If it’s true, there never was a Cam­brian Ex­plo­sion, the para­graphs above don’t cor­re­spond to the world as it was, and the Earth’s en­cephal­iza­tion took place very grad­u­ally, over eons. A re­lief in some ways (our the­ory of evolu­tion re­mains sound), but a tragedy in oth­ers (since we’d prob­a­bly never be able to peer at what might be the most im­por­tant mo­ment in the his­tory of life).

Here’s a differ­ent story:

By the end of the Protero­zoic, there has been a thriv­ing mul­ti­cel­lu­lar ecosys­tem for hun­dreds of mil­lions of years, full of com­plex an­i­mals in a thriv­ing dance of graz­ing and pre­da­tion, re­pro­duc­tion and sur­vival, grad­u­ally ex­pand­ing and ex­plor­ing the space of pos­si­ble forms. But these or­ganisms are all soft-bod­ied, with no bones or shells or rigid frame­works of any kind. When they die, their bod­ies rot im­me­di­ately, leav­ing no trace for pa­le­on­tol­o­gists to find. But re­mem­ber when I men­tioned that one of the con­ti­nents is break­ing up? All this new coastline, and new weath­er­ing, bring new min­er­als into the oceans. Sud­denly, the sys­tem is flooded with dis­solved cal­cium salts, ready and able to be in­cor­po­rated into bones and other biolog­i­cal ma­chin­ery. Nat­u­rally, a num­ber of differ­ent well-es­tab­lished species take ad­van­tage of this. Skele­tons, spines, and shells be­come very pop­u­lar. And when they die, they’re pre­served, some for a long enough for a poor fool­ish sci­en­tist to stum­ble across. And from our per­spec­tive, there’s a bright line with an­i­mals on only one side of it.

This is a pos­si­bil­ity that we should take very se­ri­ously. A good chunk of the sci­en­tific com­mu­nity cer­tainly does.

One of the things that swayed these sci­en­tists is a method of us­ing ‘molec­u­lar clocks’ to learn from liv­ing ge­netic se­quences as if they were a sort of fos­sil. The trick is this: look for ge­netic se­quences of a very spe­cific sort, those that change ran­domly, pro­tected from the di­rec­tion­al­ity of evolu­tion­ary se­lec­tion pres­sures, and which have been do­ing so at a slow and steady rate for hun­dreds of mil­lions of years. (In prac­tice, cer­tain struc­tural el­e­ments of hemoglobin work well.) Se­quence this area in two very differ­ent an­i­mal species for which we have already dis­cov­ered the age of their last com­mon an­ces­tor- and by mea­sur­ing the differ­ence, we can pre­cisely cal­ibrate the rate of change.

Then, all you have to do is ap­ply this same pro­ce­dure to, say, a mol­lusc and a chor­date. If the Cam­brian Ex­plo­sion hap­pened like we say it hap­pened, you should get about 500 mil­lion years of ge­netic drift.

Ac­tual an­swer? 800 mil­lion. This gives us a full 300 mil­lion years to go from prim­i­tive sponges to trilo­bites, an em­i­nently rea­son­able wait.

Still, I feel a lit­tle squidgy about this line of rea­son­ing. Molec­u­lar clocks are a dan­ger­ously frag­ile tool, for one. And re­mem­ber also that the 800 mil­lion year figure would have com­plex an­i­mals sur­viv­ing the snow­ball Earth pe­ri­ods, where a frozen sur­face layer pre­vented gas ex­change be­tween the oceans and at­mo­sphere; those oceans were anoxic, lack­ing any oxy­gen for the an­i­mals to breathe. But my main ob­jec­tion is ac­tu­ally a bit sim­pler: no ich­nofos­sils.

An ich­nofos­sil, or ‘trace fos­sil’ is just any biolog­i­cal rem­nant that doesn’t in­volve the ac­tual biolog­i­cal thing it­self. Foot­prints, bur­rows, etcetera. They’re kind of a pain to find- trace fos­sil hunt­ing fa­mously takes place at dawn so that the shad­ows throw your field area into sharper re­lief- but pre­serve a lot of valuable in­for­ma­tion that ac­tual bones do not. Con­sider the fa­mous ar­chaeopteryx fos­sil that pre­served, not just the skele­ton of a dinosaur like so many oth­ers, but also the clear im­print of its feathers. That is the weight of an ich­nofos­sil.

As you might imag­ine, we’ve re­ally scoured the strata around the Cam­brian tran­si­tion as best we can. For the sim­pler or­ganisms, we do find a num­ber of trace fos­sils. That’s why I can tell you about the sponges and jel­lyfish; we have the im­pres­sions they left in the ground as they died. We even have enough de­tail to learn as­ton­ish­ing things like: “Modern jel­lyfish are quadrilat­er­ally sym­met­ri­cal, a cir­cle with for iden­ti­cal quad­rants. But when they first emerged, at least some jel­lyfish were trilat­er­ally sym­met­ri­cal, a cir­cle with three iden­ti­cal parts, in­stead.”

But aside from the jel­lyfish and those weird asym­met­ri­cal fronds, we gen­uinely don’t see much. No bur­rows, no tracks, no trilobyte-shaped im­pres­sions in soft mud. So if there was this huge dy­namic ecosys­tem, why didn’t it leave any foot­prints, even though con­di­tions were fa­vor­able enough for such things that we can find the fi­nal rest­ing place of a 600 mil­lion year old jel­lyfish? The most provoca­tive traces we find are nar­row (mil­lime­ter) hori­zon­tal trails or bur­rows, just 2-3 mil­lion years be­fore the Cam­brian. That’s the only pre­lude we’ve found so far, and it’s a brief one.

I don’t think this is strong enough to break the miss­ing-cal­cium the­ory out­right; if noth­ing else, it’s an ar­gu­ment from ab­sence. But I do think it’s a pretty im­por­tant nail in the coffin, and a rea­son for skep­ti­cism. And so- we still have at least a pretty good rea­son to think that the Cam­brian Ex­plo­sion is a real thing, and that our search for the mid­dle part of that story is not fool­ish.

I can think of a few places to look.

E.

Per­haps the an­swer we’re look­ing for is, ‘oxy­gen’. As we ob­served ear­lier, this is a pe­riod in Earth’s his­tory in which oxy­gen had in­creased to near-mod­ern lev­els some­what re­cently.

Oxy­gen isn’t quite an ab­solute pre­req­ui­site for com­plex food webs, but it’s pretty close. An­i­mal metabolism de­rives en­ergy from a flow of elec­trons as they move from one molecule to an­other, al­most but not en­tirely un­like a wa­ter wheel pow­ered by a flow­ing river. The faster that wa­ter flows, the more en­er­getic a ma­chine you can power. To do that, you need a source of elec­trons, prefer­ably in some dense high-en­ergy pack­age like sugar. But you also need to provide a ground­ing, a place for them to go, some­thing that pulls in elec­trons as hard as pos­si­ble. That is, our power sys­tem also needs an elec­tron sink. Oxy­gen does this job re­mark­ably well- thus, as an­i­mals, we sur­vive by eat­ing and breath­ing.

Without oxy­gen, you need to rely on a less en­er­getic sink. There are species of iron that work alright, and there are some weird hacks available with hy­dro­gen, but the ma­chines that you can make with these power sources are only so im­pres­sive. If all you have is iron as an elec­tron sink, you can prob­a­bly man­age graz­ing pretty well, and you can buy some wig­gle room as long as you’re micro­scopic. But some­thing high-en­ergy like mul­ti­cel­lu­lar pre­da­tion is a big ask. At least to­day, when­ever we see low-oxy­gen en­vi­ron­ments in the deep ocean and so on, we always see a cor­re­spond­ing re­duc­tion in food web com­plex­ity and species di­ver­sity.

So could a quan­ti­ta­tive in­crease in oxy­gen lev­els have pro­duced a qual­i­ta­tive change in the struc­ture of an­i­mal life? Pos­si­bly. But there was a sig­nifi­cant lag be­tween the rise of oxy­gen and the rise of an­i­mal com­plex­ity, al­most a hun­dred mil­lion years. It begs the ques­tion, why not sooner? Oxy­gen prob­a­bly had some­thing to do with this, but it seems more like a pre­req­ui­site than a cause- and we have yet to ac­count for the mor­pholog­i­cal changes. Why not just use the en­ergy to make big­ger fronds? Fronds for miles, fronds to span moun­tains, fronds be­yond the wildest dreams of frond­kind!

F.

Think back to those hori­zon­tal bur­rows that show up in the very last days of the Protero­zoic. There are a cou­ple ways that this is re­ally, re­ally ex­cit­ing. First, it im­plies that sen­sa­tion prob­a­bly was start­ing to get con­cen­trated on one end of the an­i­mal, or at least one side- taste, smell, even vi­sion clus­tered around a sin­gle area. That feels sus­pi­ciously like we’re start­ing to get nerve clusters that you can al­most call a brain.

But also, sig­nifi­cantly for our pur­poses, this means that an­i­mals might be start­ing to de­velop so­phis­ti­cated hox genes.

The hox genes are, more or less, the stan­dard library for the struc­tural as­sem­bly of an­i­mal bod­ies. It’s a DNA-mod­ify­ing type of gene, one that ac­ti­vates spe­cific other re­gions of DNA dur­ing early de­vel­op­ment. Need a leg? Ac­ti­vate the ‘leg’ hox gene, and that will start a huge cas­cade of re­lated pro­cesses that make a torso seg­ment that con­tains a cou­ple legs plus all the nec­es­sary hip-joints and such in a some­what stan­dard­ized way.** This makes it eas­ier than you’d think to adapt an­i­mal body plans on the fly; rather than rein­vent­ing legs from the ground up ev­ery time you want to adapt from quadruped to hexaped, you can just have a mu­ta­tion that calls the hox gene two more times. It is also a pri­mary mechanism of di­rec­tion­al­ity, in which an­i­mal bod­ies have a clear ori­en­ta­tion with a front and back.

As you might imag­ine, an­i­mals al­most never sur­vive a mu­ta­tion to the hox genes proper, let alone thrive and spe­ci­ate. It tends to mean that you get born with­out a head or some­thing. So, both hu­mans and house flies tend to have a very similar set. That holds true across the en­tire an­i­mal king­dom, with only a few ex­cep­tions- hox genes are frozen in time, pro­por­tionate with their im­por­tance. Care to guess which types of an­i­mal lack fully formed hox genes?

That’s right: sponges and jel­lyfish. Even those have a rough proto-hox thing go­ing on, but it’s a far cry short of ours.

It goes with­out say­ing that this will have im­pli­ca­tions for species di­ver­sity in the an­i­mal king­dom. Adap­ta­tion isn’t as easy as build­ing an an­i­mal out of le­gos, but it has at least got­ten a lot eas­ier. An­i­mals can ex­per­i­ment with body shapes in more rad­i­cal ways, and be suc­cess­ful more of­ten when they do so- they’re now ex­plor­ing a much larger space of pos­si­bil­ities.

We’re start­ing to piece to­gether a rough frame­work here, in which the rise of oxy­gen and the slow de­vel­op­ment of meta-ge­nomic ad­van­tages work to­gether to provide space for a more dy­namic ecosys­tem, but is that enough to make sense of some­thing as dra­matic and sur­pris­ing as the Cam­brain Ex­plo­sion? It’s a start! But let’s see if I can’t make it a lit­tle more com­pli­cated.

**I am ly­ing harder than usual right now. This is com­pli­cated and I am not a ge­net­i­cist. Please never be­lieve me about any­thing.

G.

We’ve got the oxy­gen lev­els needed for large-scale pre­da­tion and mul­ti­lay­ered food webs, and we’ve got the ge­netic toolkit to move quickly through differ­ent an­i­mal shapes as evolu­tion­ary pres­sures come down on us, sure. But again, why so many, and why all at once? What splin­tered the an­i­mal king­dom so thor­oughly, and spread the shards of it so widely? Be­fore the Ex­plo­sion, we ap­par­ently had two or three gen­eral body plans, each with an ac­com­pa­ny­ing niche. After­wards- ev­ery­thing else.

Per­son­ally, my fa­vorite an­swer to that ques­tion is, ‘eye­balls’.

Maybe also noses and ears, I guess. But those are a lit­tle less di­rec­tional, and we’re pretty sure that eyes did in fact de­velop around this time.

Either way, what I re­ally mean is, ‘long dis­tance de­tec­tion of nu­tri­ents’. This syn­chro­nizes nicely with the di­rec­tional mo­tion that shows up around this time, the ‘front and back’ and ‘top and bot­tom’ in­no­va­tions that al­low an an­i­mal to see food and then move to­wards it.

What I re­ally re­ally mean is, ‘pre­da­tion.’

On the one hand, this makes it a lot more vi­able to be one step up in the food web. A blind preda­tor is go­ing to have some trou­ble; the abil­ity to see and pur­sue opens up a new niche. But only the one new niche, so that alone still doesn’t ex­plain the ri­o­tous di­ver­sity we new en­counter.

Con­sider the crite­ria you must satisfy to be a suc­cess­ful pre­cam­brian an­i­mal. You’re go­ing to need to ab­sorb as many com­plex car­bon molecules as pos­si­ble. You’re go­ing to have to solve the re­pro­duc­tion prob­lem some­how. And you’re go­ing to have to be struc­turally sound, rather than col­laps­ing un­der your own weight or some­thing. It’s fairly sim­ple, mostly re­volv­ing around be­ing able to ac­cess as much sea­wa­ter as pos­si­ble so you can filter or­gan­ics out of it.

And in fact, the three ma­jor solu­tions we see in the fos­sil record are, “have a large broad­side and catch wa­ter as it moves by”, “ac­tively pump wa­ter to­wards your­self”, and “move quickly through the wa­ter.” When you think about it, that’s a fairly ex­haus­tive list. Every one of these forms is clearly ex­plor­ing ways to have phys­i­cal con­tact with as much sea­wa­ter as pos­si­ble, and al­most noth­ing else. And there are only so many ways to be the best at that job.

But now let’s add an­other crite­rion: “Don’t get eaten.”

It’s not just that this is a fairly flex­ible and am­bigu­ous con­straint. It’s that we’ve lost our chance to mono­ma­ni­a­cally fo­cus on the one strat­egy of ‘con­tact lots of wa­ter’. And when you have to bal­ance differ­ent needs that are in com­pe­ti­tion, there are a few differ­ent trends that tend to emerge from that pro­cess. First, none of your strate­gies will be quite as suc­cess­ful as they were when you didn’t have to make any trade­offs. And sec­ond, there will tend to be a num­ber of differ­ent ways to bal­ance those needs, each of which is roughly as effec­tive as the oth­ers.

This is a re­ally im­por­tant but kind of ab­stract point, so I’m gonna ham­mer at it for a lit­tle bit. As the num­ber of con­straints in­creases, the num­ber of differ­ent ‘best’ solu­tions will tend to in­crease com­bi­na­tor­i­cally. If you’re a proto-sponge, and you’re sud­denly un­der all this pres­sure from be­ing hunted, you might try cov­er­ing your­self in spikes, or bor­row­ing into the mud, or grow­ing in nasty in­hos­pitable places where preda­tors don’t want to go, or just pick­ing up a cer­tain amount of mo­bil­ity. But re­mem­ber, you still have to worry about filter-feed­ing your­self. And the proto-jel­lyfish has a similar num­ber of differ­ent op­tions, and the weird frond-look­ing things too. The net re­sult is that we end up with a re­ally huge num­ber of equally vi­able body plans, rather than the two or three that are the win­ners of a sim­pler prob­lem.

Ba­si­cally, the idea here is that pre­da­tion was such a rad­i­cal change to the en­vi­ron­ment that it frac­tured a small num­ber of deep ecolog­i­cal niches in to a large num­ber of some­what shal­lower niches. The rad­i­cal spe­ci­a­tion of the Cam­brian Ex­plo­sion is sim­ply the nat­u­ral re­sult of those niches be­ing ex­plored and filled.

H.

And some­how, from that cru­cible, the brain.

We aren’t there yet. We know that long-dis­tance senses like vi­sion prob­a­bly would have given rise to di­rec­tional mo­tion and di­rec­tional anatomy, and that the com­bined needs of di­rec­tional mo­tion and of sen­sa­tion man­aged to con­cen­trate nerves in one par­tic­u­lar re­gion of the an­i­mal. Maybe we can find out more with de­tailed ge­netic analy­ses, or maybe we’ll hit the moth­er­load with some hugely im­por­tant fos­sil dis­cov­ery. Hard to say, but I doubt we’re done yet.

Analo­gies are always per­ilous, and I will per­son­ally get very an­noyed at any­one who tries to make a poli­ti­cal metaphor out of this or some­thing. But in more gen­eral terms, it’s worth tak­ing a step back and think­ing about what the Cam­brain Ex­plo­sion says about our uni­verse. The run up to this apoc­a­lypse seem to have in­cluded at least a few gen­er­al­iz­able events:

The first is a deep well of un­der­ex­ploited po­ten­tial en­ergy. Oxy­gen, in our case, meant that the ra­tio be­tween the pos­si­ble and the ac­tual was some­what larger than usual. Find­ing a metabolic path­way to ex­ploit oxy­gen was difficult, but once the blueprint ex­isted, the ma­chin­ery it­self wasn’t par­tic­u­larly difficult.

The sec­ond is that new lay­ers of use­ful ab­strac­tion emerged, which al­lowed in­no­va­tion and con­cep­tual mo­bil­ity on larger scales than had pre­vi­ously ex­isted. We ac­cel­er­ated faster through our search space.

Th­ese do not them­selves lead di­rectly to an abun­dance of new forms. Rather, the fits and starts of the early suc­cesses pre­sent new challenges to ex­ist­ing in­sti­tu­tions. Those or­ga­ni­za­tions that suc­cess­fully adapt to the new en­vi­ron­ment are still con­strained by the ad­di­tional com­plex­ity, and win­dows open for en­tirely novel forms.

And that’s about as much of a metaphor as I’m will­ing to make. Still, it prob­a­bly in­fluences a lot of the way I think about, e.g., Sili­con Valley. And it’s prob­a­bly im­por­tant to re­mem­ber that any Sin­gu­lar­ity would be the sec­ond in­tel­li­gence ex­plo­sion that the Earth has gone through.

The stro­ma­to­lites are still around, by the way. They never con­quered the world again, but they still build their lit­tle hills in hy­per­sal­ine and hy­per­ther­mal wa­ters, places where an­i­mal life can’t sur­vive. There’s a nice cluster of them in the Ba­hamas, which makes for some nice field ex­pe­di­tions for geo­biol­o­gists. It’s not a bad re­tire­ment.