Fine Read­ing: A Chalk Talk

by Roberto

As a means of com­mu­ni­cat­ing sci­ence, chalk talks can be an ex­tremely ef­fec­tive way to en­gage the au­dience. These days such pre­sen­ta­tions have be­come part and par­cel of the in­ter­view process of those seek­ing aca­d­e­mic jobs in the life sci­ences. As much as I love such chalk talks, that's not what I will dis­cuss to­day. Rather, I want to bring to your at­ten­tion a his­toric chalk talk or, bet­ter said, a talk about chalk. Let me be­gin with the ques­tion: What is chalk? First an­swer is that most "black­board chalk" (the multi-col­ored sticks we use when giv­ing chalks talks! ) is not re­ally chalk, it's gyp­sum. "Real" chalk is a soft porous rock made of CaCO₃ (cal­cite). And, strik­ingly, chalk de­posits are by and large made up coc­col­iths, the mi­cro­scopic shells of coc­col­ithophores, uni­cel­lu­lar al­gal mem­bers of phy­to­plank­ton (see also a piece in this blog ). That such mas­sive rock for­ma­tions re­sult from mi­cro­bial growth and death is com­mon knowl­edge to­day but it was not so 150 years ago. That is what makes this par­tic­u­lar talk about chalk so re­mark­able. I'm talk­ing about the lec­ture "On a Piece of Chalk" that Thomas Henry Hux­ley (a.k.a. "Darwin's Bull­dog" for be­ing such a strong ad­vo­cate of evo­lu­tion ) gave in 1868 at a meet­ing of the British As­so­ci­a­tion for the Ad­vance­ment of Sci­ence in Nor­wich. I thank Rich Lenski for alert­ing me to the ex­is­tence of this lec­ture some five years ago, af­ter I gave a lec­ture at Michi­gan State pre­sent­ing some our work on coc­col­ithophores no less! But I don't feel too bad about hav­ing been ig­no­rant of the ex­is­tence of this won­der­ful es­say. At a re­cent chalk talk that I gave, I polled the au­di­ence. Only one of the forty or so "mi­cro­bial sci­en­tists" in at­ten­dance had read it. Hope­fully, many more will read it in the fu­ture if we pass the word.

On a Piece of Chalk cover im­age Source; Thomas Henry Hux­ley pho­to­graph Source

The per­sua­sive na­ture of Huxley's nar­ra­tive is a de­light. While it is nowhere ex­plic­itly stated, I can imag­ine him with a piece of chalk in his hand, hold­ing it up and start­ing: "If a well were sunk at our feet in the midst of the city of Nor­wich, the dig­gers would very soon find them­selves at work in that white sub­stance al­most too soft to be called rock, with which we are all fa­mil­iar as chalk." He sets out to place the very fa­mil­iar writ­ing tool – a piece of chalk – in the con­text of the Earth. Quick­ly, he moves from the rock un­der­neath Nor­wich to the rock un­der­ly­ing all of Eu­rope, the Mid­dle East and Cen­tral Asia. It's all chalk! Then come two shock­ers.

First, he states that while made of a sim­ple chem­i­cal, CaCO₃, mi­cro­scopic analy­ses had shown him that chalk is made up of the cal­care­ous skele­tons of tiny ocean dwellers. Coc­col­iths he calls them, that is "round rocks." Sec­ond, he re­veals that in the process of map­ping the North At­lantic Ocean floor in prepa­ra­tion for lay­ing down of the first transat­lantic ca­ble, his friend Cap­tain Day­man has brought up sam­ples from the sea floor. Every­where be­tween Ire­land and New­found­land, the mud sam­ples, when dried out, are chalk. And when an­a­lyzed, the mud sam­ples con­tain the same cal­care­ous skele­tons and also some of their liv­ing car­ri­ers.

Many con­clu­sions that were ut­terly un­ex­pected at the time emerge from these find­ings. Nor­wich, as well as the vast ex­panse of land reach­ing into Cen­tral Asia, was once the bot­tom of an ocean. The process of chalk for­ma­tion at the bot­tom of the ocean is slow and the chalk de­posits are thou­sands of feet thick. The Earth is an­cient and yet it is slowly but con­tin­u­ally mor­ph­ing. And most ap­peal­ing to me, mi­crobes shape the Earth.

The es­say goes on to de­scribe the fos­sils of large crea­tures em­bed­ded in the chalk and in do­ing so points clearly at the slow but un­de­ni­able pace of evo­lu­tion. But, no more spoil­ers, I hope you will en­joy read­ing it in its en­tirety (for your con­ve­nience, a link to the PDF file is pro­vided be­low ). What a won­der­ful ex­am­ple of ef­fec­tive com­mu­ni­ca­tion of sci­ence! I find it very fit­ting that the most abun­dant coc­col­ithophore in present day oceans, Emil­ia­nia hux­leyi, is named in honor of the essay's au­thor.

Down­load: On a Piece of Chalk (PDF)