What is so special about the human brain? | Suzana Herculano-Houzel

Prevodilac: Jelena Jevtić Lektor: Mile Živković

What is so special about the human brain?

Zašto je toliko poseban ljudski mozak?

Why is it that we study other animals

Zašto mi proučavamo druge životinje,

instead of them studying us?

umesto da one proučavaju nas?

What does a human brain have or do

Šta to ljudski mozak ima ili radi

that no other brain does?

što nijedan drugi mozak ne može?

When I became interested in these questions about 10 years ago,

Kada sam počela da se zanimam za ova pitanja pre desetak godina,

scientists thought they knew what different brains were made of.

naučnici su mislili da znaju kakvog su sastava različiti mozgovi.

Though it was based on very little evidence,

Iako je, u osnovi, bilo malo dokaza

many scientists thought that all mammalian brains,

mnogi naučnici su mislili da su mozgovi sisara,

including the human brain,

uključujući i ljudski mozak,

were made in the same way,

potpuno isto načinjeni,

with a number of neurons that was always

sa brojem neurona koji je oduvek

proportional to the size of the brain.

bio proporcionalan veličini mozga.

This means that two brains of the same size,

To znači da dva mozga istih veličina,

like these two, with a respectable 400 grams,

kao ova dva, od pristojnih 400 grama,

should have similar numbers of neurons.

treba da imaju isti broj neurona.

Now, if neurons are the functional

Sada, ako su neuroni funkcionalne

information processing units of the brain,

prenosne jedinice informacija,

then the owners of these two brains

onda vlasnici ova dva mozga

should have similar cognitive abilities.

treba da imaju iste kognitivne sposobnosti.

And yet, one is a chimp,

A ipak, jedan vlasnik je šimpanza,

and the other is a cow.

a drugi vlasnik je krava.

Now maybe cows have a really rich

Sada, možda krave imaju jako bogat

internal mental life and are so smart

unutrašnji, duševni život i toliko su pametne

that they choose not to let us realize it,

da su odlučile da nam ne dozvole da to shvatimo,

but we eat them.

ali mi njih jedemo.

I think most people will agree

Mislim da bi se većina ljudi složila

that chimps are capable of much more complex,

da su šimpanze sposobne za mnogo složenije,

elaborate and flexible behaviors than cows are.

detaljnije i prilagodljivije ponašanje nego krave.

So this is a first indication that the

Tako da je ovo prvi pokazatelj

"all brains are made the same way" scenario

da scenario tipa: "Svi mozgovi su isto načinjeni",

is not quite right.

nije baš prihvatljiv.

But let's play along.

Ali, hajde da i mi pretpostavimo isto.

If all brains were made the same way

Ako su svi mozgovi načinjeni isto,

and you were to compare animals with brains of different sizes,

i na vama je da poredite životinje sa različitom veličinom mozga,

larger brains should always have more neurons

veći mozgovi bi uvek trebalo da imaju više neurona

than smaller brains, and the larger the brain,

od manjih i što je mozak veći,

the more cognitively able its owner should be.

veće bi trebale da budu i kognitivne sposobnosti njegovog vlasnika.

So the largest brain around should also be

Tako da najveći mozak treba da bude

the most cognitively able.

i najviše kognitivno sposoban.

And here comes the bad news:

Ali, evo loših vesti.

Our brain, not the largest one around.

Naš mozak nije najveći u okruženju.

It seems quite vexing.

Zvuči prilično uznemirujuće.

Our brain weighs between 1.2 and 1.5 kilos,

Naš mozak je težine između 1,2 i 1,5 kilograma,

but elephant brains weigh between four and five kilos,

ali mozak slona je težine od četiri do pet kilograma,

and whale brains can weigh up to nine kilos,

a mozak kita može biti težine i do 9 kilograma,

which is why scientists used to resort to saying

i zato su naučnici imali običaj da pribegavaju tvrdnji,

that our brain must be special

da naši mozgovi mora da su posebni,

to explain our cognitive abilities.

jer to objašnjava naše kognitivne sposobnosti.

It must be really extraordinary,

Mora da je zaista izvanredan,

an exception to the rule.

izuzetak od pravila.

Theirs may be bigger, but ours is better,

Njihov je možda veći, ali naš je bolji,

and it could be better, for example,

i može biti bolji, na primer,

in that it seems larger than it should be,

u tome, jer je veći nego što treba da bude,

with a much larger cerebral cortex than we should have

sa mnogo većim cerebralnim korteksom nego što bi trebalo,

for the size of our bodies.

u odnosu na veličinu našeg tela.

So that would give us extra cortex

Tako da nam ostaje višak korteksa

to do more interesting things than just operating the body.

za neke interesantnije stvari od samog funkcionisanja tela.

That's because the size of the brain

Veličina mozga

usually follows the size of the body.

obično je srazmerna veličini tela.

So the main reason for saying that

Tako da glavni razlog što kažemo

our brain is larger than it should be

da je naš mozak veći nego što bi trebalo

actually comes from comparing ourselves

u stvari dolazi iz poređenja ljudi

to great apes.

sa velikim majmunima.

Gorillas can be two to three times larger than we are,

Gorile mogu biti dva ili tri puta veće nego što smo mi,

so their brains should also be larger than ours,

tako da bi njihovi mozgovi trebalo isto tako da budu veći od naših,

but instead it's the other way around.

ali u stvari je obrnuto.

Our brain is three times larger than a gorilla brain.

Naš mozak je tri puta veći od mozga gorile.

The human brain also seems special

Ljudski mozak se isto tako čini poseban,

in the amount of energy that it uses.

i u količini energije koju koristi.

Although it weighs only two percent of the body,

Iako je težine samo dva procenta od ukupne težine tela,

it alone uses 25 percent of all the energy

koristi 25 procenata ukupne energije

that your body requires to run per day.

koju vaše telo zahteva da bi funkcionisalo u jednom danu.

That's 500 calories out of a total of 2,000 calories,

To je 500 kalorija od ukupnih 2000 kalorija,

just to keep your brain working.

samo da bi mozak bio u aktivnom stanju.

So the human brain is larger than it should be,

Znači, ljudski mozak je veći nego što bi trebalo da bude,

it uses much more energy than it should,

koristi mnogo više energije nego što je potrebno -

so it's special.

znači da je poseban.

And this is where the story started to bother me.

I evo gde je priča počela da mi smeta.

In biology, we look for rules

U biologiji, mi tražimo pravila

that apply to all animals and to life in general,

koja primenjujemo na životinje i život uopšte,

so why should the rules of evolution

i zašto se onda pravila evolucije

apply to everybody else but not to us?

primenjuju na sve ostale, ali ne i na nas?

Maybe the problem was with the basic assumption

Možda je problem bio u osnovnoj pretpostavci

that all brains are made in the same way.

da su svi mozgovi istog sastava.

Maybe two brains of a similar size

Možda, dva mozga istih veličina,

can actually be made of very different numbers of neurons.

u stvari, mogu biti sastavljeni od različitog broja neurona.

Maybe a very large brain

Možda, veoma veliki mozak

does not necessarily have more neurons

nema i veći broj neurona

than a more modest-sized brain.

od manjeg mozga.

Maybe the human brain actually has the most neurons

Možda ljudski mozak u stvari ima najviše neurona

of any brain, regardless of its size,

od bilo kog mozga, nezavisno od veličine,

especially in the cerebral cortex.

posebno u cerebralnom korteksu.

So this to me became

Tako da je ovo postalo za mene

the important question to answer:

važno pitanje na koje treba da dam odgovor:

how many neurons does the human brain have,

koliko neurona ljudski mozak ima,

and how does that compare to other animals?

i kako se to može porediti sa drugim životinjama?

Now, you may have heard or read somewhere

Možda ste čuli ili pročitali negde

that we have 100 billion neurons,

da mi imamo 100 milijardi neurona,

so 10 years ago, I asked my colleagues

i pre 10 godina, pitala sam svoje kolege

if they knew where this number came from.

da li možda znaju otkud ova brojka.

But nobody did.

Ali niko nije znao.

I've been digging through the literature

Malo sam kopala po literaturi

for the original reference for that number,

tražeći originalnu referencu za tu brojku,

and I could never find it.

i nikada je nisam mogla pronaći.

It seems that nobody had actually ever counted

Izgleda da niko nije, u stvari, nikada izbrojao

the number of neurons in the human brain,

broj neurona u ljudskom mozgu,

or in any other brain for that matter.

ili bilo kom drugom mozgu.

So I came up with my own way to count cells in the brain,

Tako da sam pronašla sopstveni način da izbrojim ćelije u mozgu,

and it essentially consists of

i u osnovi, sastoji se u

dissolving that brain into soup.

rastvaranju mozga u supu.

It works like this:

To ovako funkcioniše:

You take a brain, or parts of that brain,

mozak, ili delove mozga,

and you dissolve it in detergent,

rastvorite u deterdžentu,

which destroys the cell membranes

koji uništava ćelijske membrane

but keeps the cell nuclei intact,

ali ćelijska jezgra ostaju netaknuta,

so you end up with a suspension of free nuclei

i na kraju završite sa suspenzijom slobodnih jezgara

that looks like this,

koja izgleda ovako,

like a clear soup.

kao bistra supa.

This soup contains all the nuclei

Ova supa sadrži sva jezgra

that once were a mouse brain.

koja su nekada bila u mozgu miša.

Now, the beauty of a soup is that because it is soup,

Sada, prednosti supe su te

you can agitate it and make those nuclei

da je možete promućkati i naterati jezgra

be distributed homogeneously in the liquid,

da se pravilno rasporede u tečnosti,

so that now by looking under the microscope

tako da posmatranjem pod mikroskopom

at just four or five samples of this homogeneous solution,

samo četiri ili pet uzoraka ovog homogenog rastvora

you can count nuclei, and therefore tell

možete da izbrojite jezgra, a potom i kažete

how many cells that brain had.

od koliko ćelija se mozak sastojao.

It's simple, it's straightforward,

Jednostavno je i direktno,

and it's really fast.

i veoma brzo.

So we've used that method to count neurons

Tako da smo mi koristili tu metodu da izbrojimo neurone

in dozens of different species so far,

različitih vrsta,

and it turns out that all brains

i ispostavlja se da nisu svi mozgovi

are not made the same way.

istog sastava.

Take rodents and primates, for instance:

Uzmite glodare i primate, na primer.

In larger rodent brains, the average size

U većim mozgovima glodara, prosečna veličina

of the neuron increases,

neurona raste,

so the brain inflates very rapidly

tako da se mozak uvećava veoma brzo

and gains size much faster than it gains neurons.

i uvećava se mnogo brže nego što postiže veći broj neurona.

But primate brains gain neurons

Ali kod primata, mozak postiže veći broj neurona

without the average neuron becoming any larger,

bez uvećavanja samog neurona,

which is a very economical way

što je prilično ekonomičan način

to add neurons to your brain.

dodavanja neurona vašem mozgu.

The result is that a primate brain

Kao rezultat, mozak primata

will always have more neurons than a rodent brain of the same size,

će uvek imati više neurona od mozga glodara koji je iste veličine,

and the larger the brain,

i što je mozak veći,

the larger this difference will be.

veća će biti i razlika.

Well, what about our brain then?

A, šta je onda sa našim mozgovima?

We found that we have, on average,

Otkrili smo da mi u proseku imamo

86 billion neurons,

86 milijardi neurona,

16 billion of which are in the cerebral cortex,

16 milijardi su u cerebralnom korteksu,

and if you consider that the cerebral cortex

i ako uzmete u obzir da je cerebralni korteks

is the seat of functions like

glavni za funkcije, kao što su:

awareness and logical and abstract reasoning,

svesnost, logično i apstraktno rezonovanje,

and that 16 billion is the most neurons

i da je 16 milijardi najveći broj neurona

that any cortex has,

koji bilo koji korteks sadrži,

I think this is the simplest explanation

mislim da je ovo najjednostavnije objašnjenje

for our remarkable cognitive abilities.

naših izvanrednih kognitivnih sposobnosti.

But just as important is what the 86 billion neurons mean.

Ali isto tako važno je, šta znači uopšte broj od 86 milijardi neurona.

Because we found that the relationship

Zato što smo pronašli da veza između

between the size of the brain and its number of neurons

veličine mozga i broja neurona

could be described mathematically,

može biti matematički opisana,

we could calculate what a human brain

možemo pokazati kako bi ljudski mozak

would look like if it was made like a rodent brain.

izgledao da je po sastavu kao mozak glodara.

So, a rodent brain with 86 billion neurons

Tako da bi mozak glodara sa 86 milijardi neurona

would weigh 36 kilos.

bio težine 36 kilograma.

That's not possible.

To nije moguće.

A brain that huge would be crushed

Toliki mozak bi bio uništen

by its own weight,

svojom težinom,

and this impossible brain would go

i takav mozak bi išao

in the body of 89 tons.

uz telo težine od 89 tona.

I don't think it looks like us.

Mislim da to ne liči na nas.

So this brings us to a very important conclusion already,

Tako da nas ovo već dovodi do važnog zaključka,

which is that we are not rodents.

koje glasi: mi nismo glodari.

The human brain is not a large rat brain.

Mozak ljudi nije veliki mozak pacova.

Compared to a rat, we might seem special, yes,

U poređenju sa pacovima, mi možda izgledamo posebno,

but that's not a fair comparison to make,

ali to nije pravedno poređenje

given that we know that we are not rodents.

znajući da mi nismo glodari.

We are primates,

Mi smo primati,

so the correct comparison is to other primates.

tako da je pravilno poređenje ono sa drugim primatima.

And there, if you do the math,

I tu, ako odradite matematiku,

you find that a generic primate

pronalazite da će primat

with 86 billion neurons

sa 86 milijardi neurona

would have a brain of about 1.2 kilos,

imati mozak težine oko 1,2 kilograma,

which seems just right,

što je i sasvim dovoljno,

in a body of some 66 kilos,

za nekoga ko je težine od nekih 66 kilograma,

which in my case is exactly right,

što je u mom slučaju potpuno tačno,

which brings us to a very unsurprising

i to nas dovodi do veoma poznatog,

but still incredibly important conclusion:

ali ipak neverovatno značajnog zaključka:

I am a primate.

Ja sam primat.

And all of you are primates.

I svi vi ste primati.

And so was Darwin.

Kao što je i Darvin bio.

I love to think that Darwin would have really appreciated this.

Volim da mislim da bi Darvin zaista cenio ovo.

His brain, like ours,

Njegov mozak, kao i naš

was made in the image of other primate brains.

napravljen je po ugledu na druge primate.

So the human brain may be remarkable, yes,

Tako da ljudski mozak možda jeste izvanredan, da,

but it is not special in its number of neurons.

ali nije poseban u pogledu broja neurona.

It is just a large primate brain.

Samo je to jedan veliki mozak primata.

I think that's a very humbling and sobering thought

Mislim da je to jedna veoma skromna i trezvena misao,

that should remind us of our place in nature.

koja treba da nas podseti koje je naše mesto u prirodi.

Why does it cost so much energy, then?

Zašto je onda potrebno utrošiti toliko energije?

Well, other people have figured out

Neki ljudi su pronašli

how much energy the human brain

koliko je energije potrebno ljudskom mozgu

and that of other species costs,

i mozgu nekih drugih vrsta,

and now that we knew how many neurons

i sada kada znamo koliko neurona

each brain was made of, we could do the math.

svaki mozak ima, možemo primeniti matematiku.

And it turns out that both human

Ispostavlja se da je i za ljudske

and other brains cost about the same,

i za mozgove drugih vrsta, utrošak isti,

an average of six calories per billion neurons per day.

prosečno šest kalorija na milijardu neurona, dnevno.

So the total energetic cost of a brain

Tako da je ukupni, energetski utrošak

is a simple, linear function

jednostavna, linearna funkcija

of its number of neurons,

broja neurona,

and it turns out that the human brain

i ispostavilo se da ljudski mozak

costs just as much energy as you would expect.

troši onoliko energije koliko se i očekivalo.

So the reason why the human brain

Tako da je razlog zbog kojeg ljudski mozak

costs so much energy is simply because

troši toliko energije, jednostavan,

it has a huge number of neurons,

jer ima veliki broj neurona,

and because we are primates

i zato što smo mi primati

with many more neurons for a given body size

sa mngo većim brojem neurona u odnosu na veličinu tela

than any other animal,

od bilo koje druge vrste,

the relative cost of our brain is large,

relativna potrošnja našeg mozga je ogromna

but just because we're primates, not because we're special.

ali samo zato jer smo mi primati, ne zato što smo posebni.

Last question, then:

Poslednje pitanje, onda:

how did we come by this remarkable number of neurons,

kako smo se mi zadesili sa ovako velikim brojem neurona,

and in particular, if great apes

iako su veliki majmuni

are larger than we are,

veći od nas,

why don't they have a larger brain than we do, with more neurons?

zašto oni nemaju veći mozak, sa više neurona?

When we realized how much expensive it is

Kada smo shvatili koliko je zahtevno

to have a lot of neurons in the brain, I figured,

imati puno neurona u mozgu,

maybe there's a simple reason.

možda je razlog jednostavan.

They just can't afford the energy

Oni ne mogu da priušte dovoljno energije

for both a large body and a large number of neurons.

i za ogromna tela i za veliki broj neurona.

So we did the math.

Tako da smo odradili matematiku.

We calculated on the one hand

Izračunali smo, sa jedne strane,

how much energy a primate gets per day

koliko je energije potrebno sisaru dnevno,

from eating raw foods,

ako se hrane sirovom hranom,

and on the other hand, how much energy

i sa druge strane, koliko energije

a body of a certain size costs

za telo određene veličine.

and how much energy a brain of a certain number of neurons costs,

i koliko je energije potrebno mozgu koji ima određeni broj neurona,

and we looked for the combinations

i tragali smo za kombinacijom

of body size and number of brain neurons

veličine tela i broja neurona

that a primate could afford

koje primat sebi može priuštiti

if it ate a certain number of hours per day.

ako jede određen broj sati dnevno.

And what we found is that

I ono što smo pronašli je to

because neurons are so expensive,

da su neuroni veoma skupi.

there is a tradeoff between body size and number of neurons.

Postoji kompromis između veličine tela i broja neurona.

So a primate that eats eight hours per day

Tako da primat koji jede osam sati dnevno

can afford at most 53 billion neurons,

može najviše priuštiti 53 milijarde neurona,

but then its body cannot be any bigger

ali onda telo ne može postati teže

than 25 kilos.

od 25 kilograma.

To weigh any more than that,

Veća težina

it has to give up neurons.

znači i manji broj neurona.

So it's either a large body

Tako da ili je telo veće

or a large number of neurons.

ili je veći broj neurona.

When you eat like a primate,

Kada se hranite kao primati,

you can't afford both.

ne možete sebi priuštiti i jedno i drugo.

One way out of this metabolic limitation

Jedini izlaz iz ovakvih postavljenih metaboličkih granica

would be to spend even more hours per day eating,

bi bio: još više sati utrošiti na ishranu,

but that gets dangerous,

ali to već postaje opasno,

and past a certain point, it's just not possible.

i posle određene granice, nije ni moguće.

Gorillas and orangutans, for instance,

Gorile i orangutani, na primer,

afford about 30 billion neurons

mogu priuštiti sebi 30 milijardi neurona,

by spending eight and a half hours per day eating,

provodeći dnevno 8 i po sati u jelu,

and that seems to be about as much as they can do.

i to je izgleda i najviše što mogu.

Nine hours of feeding per day

Devet sati hranjenja dnevno,

seems to be the practical limit for a primate.

izgleda da je realna granica, koja je postavljena za primate.

What about us?

Šta je sa nama?

With our 86 billion neurons

Sa naših 86 milijardi neurona

and 60 to 70 kilos of body mass,

i od 60 do 70 kilograma telesne mase,

we should have to spend over nine hours

trebalo bi da provodimo preko devet sati

per day every single day feeding,

svakog dana, hraneći se,

which is just not feasible.

što prosto, nije izvodljivo.

If we ate like a primate,

Da jedemo kao primati,

we should not be here.

ne bi trebalo da budemo ovde.

How did we get here, then?

Kako smo u stvari dospeli ovde, onda?

Well, if our brain costs just as much energy

Pa, ako našim mozgovima treba energije

as it should, and if we can't spend

koliko je potrebno, i ako ne možemo da provedemo

every waking hour of the day feeding,

svaki sat tokom dana, hraneći se

then the only alternative, really,

onda je jedina mogućnost, zaista

is to somehow get more energy

da nekako dobijemo više energije

out of the same foods.

iz iste hrane.

And remarkably, that matches exactly

I izvanredno, to se poklapa upravo

what our ancestors are believed to have invented

sa otkrićem naših predaka

one and a half million years ago,

pre milion i po godina,

when they invented cooking.

kada su i izumeli kuvanje.

To cook is to use fire

Kuvanje znači koristiti vatru

to pre-digest foods outside of your body.

da bi se hrana mogla bolje svariti.

Cooked foods are softer, so they're easier to chew

Kuvana hrana je mekša, tako da je lakša da se žvaće

and to turn completely into mush in your mouth,

i potpuno se pretvara u kašu u ustima,

so that allows them to be completely digested

tako da može biti potpuno svarena

and absorbed in your gut,

i apsorbovana u stomaku,

which makes them yield much more energy in much less time.

što za kraće vreme donosi mnogo više energije u mnogo kraćem vremenu.

So cooking frees time for us to do

Tako da kuvanje nama štedi vreme da bismo radili

much more interesting things with our day

neke mnogo interesantnije stvari u našem danu

and with our neurons

i sa našim neuronima,

than just thinking about food,

da ne bismo samo razmišljali o hrani,

looking for food, and gobbling down food

tragali za hranom i gutali hranu

all day long.

po čitav dan.

So because of cooking, what once was

I tako, zbog kuvanja, ono što je

a major liability, this large,

nekada bila velika odgovornost, ovako veliki

dangerously expensive brain with a lot of neurons,

veoma skup mozak sa puno neurona,

could now become a major asset,

sada može postati naše glavno oruđe,

now that we could both afford the energy for a lot of neurons

sad kada možemo priuštiti i energiju za puno neurona

and the time to do interesting things with them.

i vreme, da bismo uradili zanimljive stvari sa njima.

So I think this explains why the human brain

Tako da ja mislim ovo objašnjava zašto je ljudski mozak

grew to become so large so fast in evolution,

tako brzo postao razvijen tokom evolucije,

all of the while remaining just a primate brain.

a sve vreme ostajući samo mozak primata.

With this large brain now affordable by cooking,

Sa ovako velikim mozgom, kojeg možemo i koristiti, zahvaljujući kuvanju,

we went rapidly from raw foods to culture,

munjevito smo prešli sa sirove hrane na kulture,

agriculture, civilization, grocery stores,

poljoprivredu, civilizaciju, bakalnice,

electricity, refrigerators,

elektricitet, frižidere,

all of those things that nowadays

sve te stvari koje nam danas

allow us to get all the energy we need

omogućavaju da priuštimo svu potrebnu energiju

for the whole day in a single sitting

za ceo dan, samo u jednom obroku

at your favorite fast food joint.

u omiljenom lancu brze hrane.

So what once was a solution

Tako da, ono što je nekada bilo rešenje,

now became the problem,

sada postaje problem,

and ironically, we look for the solution in raw food.

i ironično, to rešenje tražimo u sirovoj hrani.

So what is the human advantage?

I šta je prednost ljudi?

What is it that we have

Šta je to što mi imamo,

that no other animal has?

a nijedna druga životinja nema?

My answer is that we have the largest number

Moj odgovor je da imamo najveći broj

of neurons in the cerebral cortex,

neurona u cerebralnom korteksu,

and I think that's the simplest explanation

i mislim da je to najjednostavnije objašnjenje

for our remarkable cognitive abilities.

za naše izuzetne kognitivne sposobnosti.

And what is it that we do that no other animal does,

A šta je ono što mi radimo, a nijedna druga životinja ne radi

and which I believe was fundamental

i što mislim da je bilo fundamentalno

to allow us to reach that large,

da bismo uopšte mogli da dosegnemo taj veliki,

largest number of neurons in the cortex?

najveći broj neurona u moždanoj kori?

In two words, we cook.

Dve reči: mi kuvamo.

No other animal cooks its food. Only humans do.

Nijedna druga životinja ne kuva svoju hranu. Samo ljudi to rade.

And I think that's how we got to become human.

I ja mislim da smo upravo tako mi i postali ljudi.

Studying the human brain changed the way I think about food.

Proučavanje ljudskog mozga je promenilo način na koji razmišljam o hrani.

I now look at my kitchen,

Sada pogledam u svoju kuhinju

and I bow to it,

i klanjam se,

and I thank my ancestors for coming up

i zahvaljujem našim precima što su nabasali

with the invention that probably made us humans.

na otkriće koje nas je verovatno i načinilo ljudima.

Thank you very much.

Hvala vam mnogo.

(Applause)

(Aplauz)