LUCY CAIU DA ÁRVORE

Conta a lenda que, na noite de $24$ de novembro de $1974$, as estrelas brilhavam na beira do rio Awash, no interior da Etiópia. Um gravador K7 repetia a música dos Beatles “Lucy in the Sky with Diamonds”. Inspirados, os paleontólogos decidiram que a fêmea AL 288-1, cujo esqueleto havia sido escavado naquela tarde, seria apelidada carinhosamente de Lucy.

Lucy tinha $1,10$ metros e pesava $30kg$. Altura e peso de um chimpanzé. Mas não se iluda, Lucy não pertence à linhagem que deu origem aos macacos modernos (1). Ela já andava ereta sobre os membros inferiores. Lucy pertence à linhagem que deu origem ao animal que escreve esta crônica e ao animal que a está lendo, eu e você.

Os ossos foram datados. Lucy morreu $3,2$ milhões de anos atrás. Ela viveu $2$ milhões de anos antes do aparecimento dos primeiros animais do nosso gênero, o Homo habilis. A enormidade de $3$ milhões de anos separa Lucy dos mais antigos esqueletos de nossa espécie, o Homo sapiens, que surgiu no planeta faz meros $200$ mil anos. Lucy, da espécie Australopithecus afarensis, é uma representante das muitas espécies que existiram na época em que a linhagem que deu origem aos homens modernos se separou da que deu origem aos macacos modernos. Lucy já foi chamada de elo perdido, o ponto de bifurcação que nos separou dos nossos parentes mais próximos (2).

Uma das principais dúvidas sobre a vida de Lucy é a seguinte: ela já era um animal terrestre, como nós, ou ainda subia em árvores?

Muitos ossos de Lucy foram encontrados quebrados, seus fragmentos espalhados pelo chão (3). Até agora, se acreditava que isso se devia ao processo de fossilização e às diversas forças às quais esses ossos haviam sido submetidos. Mas os cientistas resolveram estudar em detalhes as fraturas.

As fraturas, principalmente no braço, são de compressão, aquela que ocorre quando caímos de um local alto e apoiamos os membros para amortecer a queda. Nesse caso, a força é exercida ao longo do eixo maior do osso, causando um tipo de fratura que é exatamente o encontrado em Lucy. Usando raciocínios como esse, os cientistas foram capazes de explicar todas as fraturas a partir da hipótese de que Lucy caiu do alto de uma árvore de pé, se inclinou para frente e amortizou a queda com o braço.

Uma queda de $20$ a $30$ metros e Lucy atingiria o solo a $60km/h$, o suficiente para matar uma pessoa e causar esse tipo de fratura (4). Como existiam árvores dessa altura onde Lucy vivia e muitos chimpanzés sobem até $150$ metros para comer, uma queda como essa é fácil de imaginar.

A conclusão é que Lucy morreu ao cair da árvore. E se caiu era porque estava lá em cima. E se estava lá em cima era porque sabia subir. Enfim, sugere que Lucy habitava árvores.

Mas na minha mente ficou uma dúvida. Quando criança, eu subia em árvores. E era por não sermos grandes escaladores de árvores que eu e meus amigos vivíamos caindo, alguns quebrando braços e pernas. Será que Lucy morreu exatamente por tentar fazer algo que já não era natural para sua espécie?

(Fernando Reinach. Adaptado de O Estado de S. Paulo, 24/09/2016.)

Recent human adaptations

Human populations live in an extraordinary variety of different habitats: hot and cold, wet and dry; in forests, grasslands and tundra. Different human groups feed on a wide variety of food sources. For many populations, diets shifted further with the development of agriculture in the past 10,000 years. To what extent have these and other factors led to genetic adaptation?

¹Human populations differ in various phenotypes – observable characteristics that result from interactions between genes and the environment –, but ²scientific studies have shown that phenotypic differences have a genetic basis and are adaptive. For example, mammals that live in cold climates tend to have larger, rounder bodies and shorter limbs than members of the same or closely related species in warm climates. ³These patterns do appear to also hold in humans, implying that population movements into colder climates were accompanied by adaptation to larger, stockier body shape, presumably to improve thermal efficiency. At the other end of the spectrum is the pygmy phenotype that has evolved in rainforest populations in Africa, South-East Asia and South America. Research has suggested that ⁴this phenotype may be an adaptation to food limitations, high humidity or dense forest undergrowth.

Another impressive example of adaptation is provided by human populations living at high altitude, especially in the Himalayas and the Andes. Compared to related lowland populations, ⁵these high elevation populations show a group of physiological adaptations to low oxygen. These adaptations include markedly increased blood flow and oxygen delivery to the uterus during pregnancy, substantially reducing the risk of babies with low birthweight. ⁶Current evidence suggests that these differences are not simply the result of recent acclimation, but are at least partly genetic. If this is the case, then ⁷the adaptation must have occurred rapidly, because ⁸these high altitude regions were settled within the last 10,000 years.

⁹Skin pigmentation is perhaps the phenotype that varies most obviously among human populations. ¹⁰ Dark pigmentation is strongly associated with tropical climates, and the spread of prehistoric humans into northern latitudes was accompanied by a shift to lighter skin color. We now know of at least half a dozen different genes that affect skin, hair or eye pigmentation. In particular, the evolution of light skin color occurred largely in parallel in western Eurasia and east Asia, but we still know few of the relevant genes in east Asia. Adaptation to lighter pigmentation may have been motivated by a need to increase UV absorption for vitamin D synthesis at high latitudes or by sexual selection.

These are only a few cases of genetic adaptation. There are surely some – perhaps many – other ¹¹factors yet to be found.

(sciencedirect.com)

Recent human adaptations

Human populations live in an extraordinary variety of different habitats: hot and cold, wet and dry; in forests, grasslands and tundra. Different human groups feed on a wide variety of food sources. For many populations, diets shifted further with the development of agriculture in the past 10,000 years. To what extent have these and other factors led to genetic adaptation?

¹Human populations differ in various phenotypes – observable characteristics that result from interactions between genes and the environment –, but ²scientific studies have shown that phenotypic differences have a genetic basis and are adaptive. For example, mammals that live in cold climates tend to have larger, rounder bodies and shorter limbs than members of the same or closely related species in warm climates. ³These patterns do appear to also hold in humans, implying that population movements into colder climates were accompanied by adaptation to larger, stockier body shape, presumably to improve thermal efficiency. At the other end of the spectrum is the pygmy phenotype that has evolved in rainforest populations in Africa, South-East Asia and South America. Research has suggested that ⁴this phenotype may be an adaptation to food limitations, high humidity or dense forest undergrowth.

Another impressive example of adaptation is provided by human populations living at high altitude, especially in the Himalayas and the Andes. Compared to related lowland populations, ⁵these high elevation populations show a group of physiological adaptations to low oxygen. These adaptations include markedly increased blood flow and oxygen delivery to the uterus during pregnancy, substantially reducing the risk of babies with low birthweight. ⁶Current evidence suggests that these differences are not simply the result of recent acclimation, but are at least partly genetic. If this is the case, then ⁷the adaptation must have occurred rapidly, because ⁸these high altitude regions were settled within the last 10,000 years.

⁹Skin pigmentation is perhaps the phenotype that varies most obviously among human populations. ¹⁰ Dark pigmentation is strongly associated with tropical climates, and the spread of prehistoric humans into northern latitudes was accompanied by a shift to lighter skin color. We now know of at least half a dozen different genes that affect skin, hair or eye pigmentation. In particular, the evolution of light skin color occurred largely in parallel in western Eurasia and east Asia, but we still know few of the relevant genes in east Asia. Adaptation to lighter pigmentation may have been motivated by a need to increase UV absorption for vitamin D synthesis at high latitudes or by sexual selection.

These are only a few cases of genetic adaptation. There are surely some – perhaps many – other ¹¹factors yet to be found.

(sciencedirect.com)

The text “Lucy caiu da árvore” is about an ancestral African female. Her characteristics can be related to the studies on phenotypes presented in the text “Recent human adaptations”.

Among her characteristics, the ones that best illustrate one of these studies are: