And
in true biker-chick fashion, Ardi chews up and spits out conventional
thinking about hominid origins, according to a team — led by
anthropologist Tim White of the University of California, Berkeley —
that unearthed and analyzed her fragile bones (
SN: 10/24/09, p. 9).
First, White and his colleagues assert, Ardi’s unusual mix of apelike
and monkeylike traits demolishes the long-standing assumption that
today’s chimpanzees provide a reasonable model of either early hominids
or the last common ancestor of people and chimps — an ancestor which
some scientists suspect could even have been Ardi, if genetics-based
estimates of when the split occurred are borne out.
Second, the team concludes, Ardi trashes the idea that
knuckle-walking or tree-hanging human ancestors evolved an upright gait
to help them motor across wide ancient savannas. Her kind lived in
wooded areas and split time between lumbering around on two legs
hominid-style and cruising carefully along tree branches on grasping
feet and the palms of the hands.
One member of White’s team argues for a controversial possibility: that two-legged walking evolved because
Ardipithecus
males had small canine teeth. Many living and fossil male apes fight
for mates by wielding formidable canines, but Ardi’s male counterparts
had to band together and forage over long distances to obtain mates, his
thinking goes.
In a third slap at scientific convention, Ardi fits a scenario in
which a few closely related hominid lineages preceded the larger-brained
Homo genus that emerged around 2.4 million years ago, White
says. In contrast, many anthropologists think of hominid evolution as a
bush composed of numerous lineages that, for the most part, died out.
Each of Ardi’s challenges draws plenty of fire. While lauding the new
finds and the painstaking reconstruction of Ardi’s bony frame, some
critics dismiss White and company’s reading of the fossils as incomplete
and speculative.
Presentations at the Royal Society of London in October by several
members of the Ardi excavation team produced “much sparring,” says
anthropologist William McGrew of the University of Cambridge in England.
“There’s legitimate disagreement,” White says. “But Ardi provides a
perspective on early hominid evolution that was previously missing. This
is a really bizarre primate.”
Chimp change
Ardi ostenta um medley esquelético peculiar que empurra chimpanzés e gorilas para fora dos holofotes evolutivos, diz o antropólogo Owen Lovejoy, membro da equipe de White. Os restos antigos de Ardi indicam que o último ancestral comum de humanos e chimpanzés não deve ter se parecido com chimpanzés vivos, como muitos pesquisadores assumiram, afirma Lovejoy, da Kent State University, em Ohio.
Desde uma cisão de 8 milhões de anos atrás, os chimpanzés e gorilas evoluíram ao longo de caminhos evolutivos que eventualmente produziram características especializadas, como caminhar com os dedos, diz ele.
Na sua opinião, Ardi indica que um ancestral de chimpanzé humano tinha proporções e pés semelhantes aos de um macaco, uma região lombar flexível e sem chimpanzés e a capacidade de se mover ao longo dos galhos das árvores de quatro em vez de balançar no estilo chimpanzé de galho em galho e pendurar pelos braços estendidos.
"Ardipithecus, não chimpanzés vivos, oferece uma perspectiva notavelmente boa sobre o último ancestral comum", diz ele. "Não podemos modificar a verdade para tornar os chimpanzés mais importantes."
Essa conclusão deixa alguns cientistas impressionados. "É muito cedo para afirmar que sabemos como era o último ancestral comum sem realmente encontrar seus fósseis", comenta o antropólogo Brian Richmond, da Universidade George Washington, em Washington, DC.
Richmond sustenta que Ardi viveu vários milhões de anos após o último ancestral comum, tempo de sobra para sua espécie ter evoluído mudanças esqueléticas substanciais.
And those changes may not have been as substantial as White’s team
claims, adds Richmond. Ardi’s curved toes, wide big toe and large body
correspond pretty well to chimps, in his opinion.
Other fossil evidence suggests that hominids came from a climbing and
knuckle-walking ape ancestor that was unlike Ardi, Richmond argues.
Chimps and other living apes can provide testable ideas about issues
such as tool use among early hominids and even the last common ancestor (
SN: 11/21/09, p. 24), McGrew says. “Ardi is an intermediate hominid form, as is Lucy. So what?” he asks.
Questions remain about whether Ardi had the build for regular upright
walking — a clear marker of hominid status — or for primarily moving
through trees, with occasional two-legged jaunts on the ground, adds
anthropologist John Hawks of the University of Wisconsin–Madison.
Consider
Oreopithecus, an ape that lived on an island near
Italy between 9 million and 7 million years ago. This creature possessed
a pelvis, legs and feet that supported tree climbing as well as slow
and somewhat stilted walking.
“
Oreopithecus shows that there are alternate pathways to evolving a ground-based skeleton from the ape body plan,” Hawks says.
But
Oreopithecus differed from Ardi in critical ways, Lovejoy responds, such as having extremely long arms. “Locomotion differed vastly between
Oreopithecus and Ardi,” he says.
If
Ardipithecus adopted upright walking in a big way and was
a precursor of the human lineage, Hawks posits, “it could be the first
hominid or perhaps even the common ancestor of humans and chimps — if we
take genetic studies seriously.” DNA analyses suggest that people and
chimps split from a common ancestor between 5 million and 4.5 million
years ago, around Ardi’s time.
Lovejoy regards those genetic estimates as unreliable. DNA studies
rest on doubtful assumptions, he says, such as constant rates of genetic
mutation in the human and chimp lineages.Fossil evidence places the
human-chimp split at more like 8 million to 10 million years ago, in his
view.
Hominid family values
Disputes over Ardi’s evolutionary relationships to living and extinct
apes seem cordial compared with debate over her sexual relationships
and their implications for ancient hominid social life.
This fracas goes back to 1981, when Lovejoy published a paper in
Science about the sex life of what was, at that time, the earliest known hominid species,
Australopithecus afarensis. The most famous member of that
species is Lucy, a 3.2-million-year-old partial female skeleton found at
another Ethiopian site in 1974. Lovejoy proposed that Lucy’s kind
possessed traits consistent with what amounted to a sexual revolution in
the ape world (
SN: 6/11/05, p. 379).
Na maioria das espécies de macacos, os machos são muito maiores que as fêmeas e lutam violentamente para acasalar com fêmeas férteis, que anunciam sua disponibilidade com tecido vermelho inchado. As fêmeas criam filhos por conta própria.
Lucy amou o acordo, argumentou Lovejoy. Os machos cresceram apenas um pouco maiores que as fêmeas e tinham caninos pequenos. Os adultos de ambos os sexos favoreciam os relacionamentos de longo prazo como questão de sobrevivência, ele teorizou. Os machos forneciam comida para parceiros regulares com quem tinham filhos, permitindo que as fêmeas passassem mais tempo criando seus próprios filhos.
A monogamia funcionou, na visão de Lovejoy, porque a anatomia feminina evoluiu para mascarar sinais óbvios de ovulação que sinalizam prontidão sexual para os homens, desenvolvendo características como seios permanentemente aumentados. Os sindicatos de sucesso conseguiram uma boa chance de não ter filhos e, assim, tornaram-se desagradáveis para ambos os sexos.
As evidências de Lovejoy sobre diferenças mínimas de tamanho entre os sexos de A. afarensis foram fortemente criticadas. Os críticos afirmam que ele subestimou as disparidades de tamanho.
Detratores acrescentam que machos na posição vertical com caninos diminutos poderiam ter encontrado muitas maneiras de se esmurrar nas batalhas de acasalamento, mesmo que tivessem que recorrer a socos.
With Ardi in tow, Lovejoy has now elaborated on his argument. A
transition to monogamous relationships, expanded child care by mothers
and hidden female ovulation first occurred before Lucy, in
Ardipithecus, he proposes. Ardi’s kind displays even smaller sex differences in canine size than Lucy’s species. “
Australopithecus represents a more intense version of what was already evolving in
Ardipithecus,” Lovejoy says.
Cooperation among males later expanded in
A. afarensis, he posits. Male bands scoured forests and savannas for food and worked together to avoid and defend against predators.
A social puzzle
Ardipithecus canines excavated by White’s team validate
Lovejoy’s scenario, remarks anthropologist Robert Tague of Louisiana
State University in Baton Rouge. Male canines are slightly larger than
those of females, but all the canines are about the size of female
chimps’ canines, he says.
“Although Lovejoy’s theory is widely cited and presented in almost
all biological anthropology textbooks, it is also widely rejected,”
Tague acknowledges.
And for good reason, argues J. Michael Plavcan of the University of
Arkansas in Fayetteville. Using a different statistical approach, he
estimates that Lucy was actually considerably smaller than her male
cohorts.
To portray early hominids as a peaceful, monogamous crowd “is
phenomenally speculative,” Plavcan says. Although large-bodied primate
males with fanglike canines usually fight over mates, minimal sex
differences can result in any of a variety of mating arrangements, he
contends.
What’s more,
Ardipithecus ramidus fossils do display size
differences between the sexes sufficient to assume that males mated with
several females, as in many other primates with size disparities,
McGrew remarks.
“Lovejoy’s social hypothesis is an interesting just-so story,”
Richmond asserts. “He’s winning the competition for the title of the
Rudyard Kipling of paleoanthropology.”
Primatologist Frans de Waal of Emory University in Atlanta doesn’t
dismiss Lovejoy’s social hypothesis but faults him for comparing Ardi’s
kind with common chimps while ignoring pygmy chimps, or bonobos. Bonobos
have small canines relative to common chimps, a largely peaceful social
life and a fondness for sexual activity.
“It’s high time for a new look at the bonobo,” de Waal wrote in a
published commentary shortly after the Ardi papers appeared in
Science. “What if we descend not from a blustering chimplike ancestor but from a gentle, empathic, bonobo-like ape?”
That’s doubtful, since bonobos differ in some critical ways from
Ardi’s kind, Lovejoy responds. In particular, he says, bonobo males
display moderately larger canines and body sizes than females.
“
Ardipithecus ramidus preserves some of the ancestral
characteristics of the last common ancestor [of humans and chimps] with
much greater fidelity than does any living African ape,” Lovejoy says.
Not-so-bushy evolution
If Ardi cuts a singular figure that sets her apart from living apes,
she also bolsters an argument for cutting back the expanding number of
proposed early hominid lineages, White says. Since 1994, fossil
discoveries have led to reports of four new genera from eastern Africa
and Chad: 7-million to 6-million-year-old
Sahelanthropus (
SN: 7/13/02, p. 19), 6-million-year-old
Orrorin (
SN: 7/14/01, p. 20), 3.5-million-year-old
Kenyanthropus (
SN: 3/24/01, p. 180) and
Ardipithecus, including fragmentary remains of 5.8-million to 5.2-million-year-old
Ardipithecus kadabba.
White’s team folds
Sahelanthropus, known only from skull remains, and
Orrorin, known from fossil teeth and leg-bone pieces, into the better-described
Ardipithecus genus.
“
Ardipithecus may represent a long period of stasis in hominid evolution,” Lovejoy says.
From about 6 million to 4.2 million years ago, he proposes,
Ardipithecus evolved as a set of separate hominid groups in East Africa that interbred enough to maintain biological unity.
After that, Ardi’s kind possibly evolved into the first
Australopithecus species. Or, one
Ardipithecus group may have settled in an isolated area where it alone evolved into
Australopithecus. It’s also possible that
Australopithecus derived from a hominid lineage that researchers haven’t found, relegating
Ardipithecus to an evolutionary side branch.
Anthropologists, in particular those who have excavated and named other early hominid genera, have not jumped on the
Ardipithecus
bandwagon. Proponents of bushy hominid evolution, such as Richmond,
rely on computerized models that divvy up species by distinguishing
between shared and distinctive skeletal traits across fossil sets, an
approach that White and Lovejoy have criticized (
SN: 11/25/00, p. 346).
“More time is needed to study Ardi and compare her to living
primates,” Hawks says. “White’s team had 15 years to study this skeleton
that the rest of us saw for the first time in October.”
Complaints have circulated in anthropological circles over the past
decade that White has inappropriately kept outside investigators from
studying Ardi’s remains. White vehemently denies those charges, saying
that he has abided by Ethiopian law by publishing an initial description
of the finds before making them available for others to study.
Researchers can now examine casts of the
Ardipithecus fossils or, in certain cases, the fragile bones themselves, White says.
“These finds are phenomenally important and will keep many of us busy
for years to come,” says anthropologist Carol Ward of the University of
Missouri in Columbia.
In other words, the evolutionary shindig that Ardi crashed has just started. The night is young. Party hearty, Ardi.
