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PERSPECTIVES

of the heavy and light chains that constitute

each antibody. Elements from sets of possible

gene regions recombine to encode the heavy

and light chains, allowing a vast array of com-

binatorial possibilities. Each antibody carries

a complementarity determining region (CDR)

that complements an epitope on a target pro-

tein. An antibody-producing B cell begins

to proliferate upon recognition of a comple-

menting epitope. During this expansion, in

a process called affinity maturation, somatic

mutations are focused in the CDR coding

regions, generating many slight variants of the

antibody. B cells with mutations that improve

antibody affinity for its target protein are

selected as the immune response matures.

Through modeling of antibody sequences,

the lineages leading to effective antibod-

ies can be reconstructed, and Wu et al. and

Scheid et al. show that VRC01-like antibod-

ies favor particular variable genes that encode

the CDR. Scheid et al. found that each of 10

CD4bs antibodies isolated from five differentpatients used one of two closely related germ-

line heavy-chain genes, VH1-2or VH1-46,

and certain light-chain genes were favored;

Wu et al. also identified the common precur-

sor VH1-2. Both studies show that the pre-

dicted unmutated ancestor for these B cells

had weak or no apparent affinity for HIV Env,

and subsequent somatic mutation acquisi-

tion was essential for the breadth and potency

of these antibodies. Variants of VRC01-like

antibodies were common in individuals with

BNAb responses. Scheid et al. found multiple

VRC01-like antibodies among B cell clonesisolated from four HIV-infected individuals.

Wu et al. explored the diversity of the B cells

from two donors in depth through pyrose-

quencing and uncovered many highly mutated

variant sequences of the same lineage (orig-

inating from a common heavy-chain gene,

IGHV1-2*02).

Wu et al. and Scheid et al. obtained struc-

tures by x-ray crystallography to deduce pre-

cise regions of target recognition for repre-

sentative BNAbs. The structures showed that

the antibodies exhibited a highly convergent

mode of binding to the CD4bs epitope of

gp120 (an Env constituent) despite extensivesequence variation in the antibody CDRs.

The BNAbs recognized the same epitope,

and the same selective forceaffinity for the

CD4bsdrove antibody evolution. Thus, the

sequences evolved to find distinct answers

to recognition of a structurally conserved,

but still somewhat variable, epitope (see the

first f igure). Affinity maturation is associ-

ated with reduced entropic cost of antibody-

antigen binding (11); fine-tuning of hydro-

phobic interactions in the BNAbs reported

Fossilized Feathers

PALEONTOLOGY

Mark A. Norell

The evolution of feather color, structure, and organization is gleaned from amber-preserved specim

Not long ago, extinct dinosaurs were

considered by most as scaly and

dull. All the known fossils of primi-

tive birds (stem avians) could easily fit on a

desk and our only look at Mesozoic [250 to

65 million years ago (Ma)] feathers (except

for a few isolated plumes) wasArchaeop-teryx, a theropod dinosaur considered by

most to be the most primitive bird (see the

figure). How things have changednow

it would take a warehouse to store all the

feathered Mesozoic stem birds and nona-

vian dinosaurs that have been collected from

global deposits. Feathered animals abound

and extend deep into nonavian history

even, perhaps, to the base of dinosa

themselves. Now, instead of scaly anim

portrayed as usually drab creatures, we h

solid evidence for a fluffy colored past. T

reports in this issue, by McKellar et al.

on page 1619 and Wogelius et al. (2) on p

1622, provide a glimpse of the color patteing in the feathers of ancient birds.

Despite many reports over the past dec

of feathered dinosaurs and new birds fr

China, only now are we beginning to und

stand just how diverse feather types wer

the Mesozoic (3, 4). The feathers of tod

birds are incredibly complex structures

For decades their origins were a conundr

for evolutionary biologists, even lead

Charles Darwin to proclaim The sight o

feather in a peacocks tail, whenever I gDivision of Paleontology, American Museum of Natural His-tory, New York, NY 10024, USA. E-mail: norell@amnh.org

by Wu et al. may be the basis for increased

affinity. VRC01 is not polyreactive (4, 12),

although this is common among BNAbs (5).

Scheid et al. found, however, that 65% of the

related antibodies they isolated were polyre-

active, although the germline antibodies they

tested were not polyreactive. Thus, the struc-

tural plasticity of the antibodies that mediated

polyspecificity (13) evolved in conjunction

with high affinity for the CD4bs.

Scheid et al. identified common sequence

features among VRC01-like antibodies. The

contact residues in VRC01 that are the most

highly conserved among VRC01-like anti-

bodies from both studies directly interact

with highly conserved residues in gp120 of

Env (see the second figure). By contrast, con-

tact residues that are more variable among

VRC01-like antibodies interact at the periph-

ery of the CD4bs, where residues are less well

conserved. Thus, key conserved contacts may

be a critical feature of the epitope-antibody

interaction that enables neutralization breadth.Detailed reconstruction of the natural

development of BNAbs suggests a new direc-

tion for vaccine design: choosing vaccine

antigens that trigger ancestral states of anti-

bodies. This is a concept thats time has come.

While Wu et al. and Scheid et al. have created

a detailed portrait of the evolution and nature

of VRC01-like antibodies, parallel investi-

gations have mapped the B cell lineages of

other promising BNAbs that target differ

regions on HIV Env (1416). The gene

approach may be useful for reliably stimu

ing an initial B cell response from a nave r

ertoire when the unmutated ancestor has a

affinity for HIV Env. The logical extension

thisluring an antibody down a control

affinity maturation pathway by providing a

gens that stimulate intermediary statesa

becomes an intriguing and hopeful possibi

References

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science.1207532.

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2/2www.sciencemag.org SCIENCE VOL 333 16 SEPTEMBER 2011

PERSPECTIV

at it, makes me sick! (6) when he contem-

plated the evolution of these structures. The

reason for Darwins dilemma was that theearliest and most primitive feathers found in

association withArchaeopteryxwere already

of modern aspect. The subsequent discov-

ery of several new types of feathers (7) led

to an evolutionary series created purely on

the basis of phylogenetic information (based

on morphology). This series of simple bris-

tles to complex fans agrees nicely with inde-

pendently developed models suggestive of

several stages of feather organization during

their evolution (5, 8).

The amber-preserved fossils described

by McKellar et al. from the Late Cretaceousof Canada (about 70 Ma) show that a diver-

sity of primitive and advanced feather mor-

phologies were present in a Late Cretaceous

ecosystem. Earlier examples of feathers

from Late Cretaceous (90 to 94 Ma) amber

(9) did not display such diversity. Amber

preserves not only the microstructure, but

actual visual color as wellfeatures not

preserved in typical compression fossils.

All of the feather organizational types are

represented in the fossils described by

McKellar et al.: primitive stage 1 filaments

like those from China most commonly asso-

ciated with compsognathids and tyranno-saurids; stage 2 clusters found in all sorts

of theropod dinosaurs but best known in

dromaeosaurs; and other feather types typi-

cal of modern birds. Most unexpected is a

feather with barbules (the structures that act

like Velcro to keep feathers self-organized

and neat) that are tightly coiled around their

bases. In modern birds these are associated

with diving behavior in birds such as grebes.

These indicate that the diversity of feathers

first identified in the Late Jurassic (160 to

145 Ma) and Early Cretaceous not unexpect-

edly continues to the Late Cretaceous and

that complex modern feather adaptationshad already appeared before the extinction

of the nonavian dinosaurs.

Wogelius et al. show how new technol-

ogy can detect minute geochemical traces

and reveal the coloring of these animals.

Color in extant bird feathers is conferred by

several factors, including pigmentation and

structure. These are somewhat intertwined,

but pigment is primarily caused by special-

ized cells (melanosomes) that concentrate

organometallic compounds (compounds

like carotenoids may also influence color).

Other color patterns (like the iridescentgorgets of male hummingbirds) are due to

structural characteristics; light reflected by

the feathers causes interference effects, pro-

ducing flashy colors.

In a few cases, excellent preservation

allows comparison of the shape of fossil

melanosomes with those in extant bird feath-

ers. In the Mesozoic, these have primarily

focused on the Early Cretaceous bird Confu-

ciusornis(10) and a few nonavian dinosaurs

[likeAnchiornis(11) and Sinosauropteryx],

which are closely related to modern birds.

These studies suggested that Confuciusor-

nis(10) had a rather ruddy reddish topcolorand some countershading, that Sinosaurop-

teryxhad a reddish banded tail, and that

Anchiornis was almost woodpecker-like

with a black body, banded wings, and red-

dish head comb.

In living birds, melanosomes are of two

types: Eumelanosomes are dark brown or

black, and phenomelanosomes are reddish

brown. Wogelius et al. found dense popula-

tions of eumelanosomes across the back and

wings, and even evidence of banding on the

proximal wing, but there was no evidenc

for the presence of phenomelanosomes i

any of the samples they looked at, includ

ing several fossil feathers and a fossil squid

This is surprising because other studies o

feathered dinosaurs and even the recen

study of a much younger fossil pengui

report positive evidence for the presence o

phenomelanosomes (12).

However, even though the lack of identi

fication of phenomelanosomes is troubling

Wogelius et al. raise the interesting prospec

that feather coloration may be determine

even when physical (e.g., visual) evidenc

for feathers (or even melanosomes) is lack

ing because what is being mapped are trac

organometallic biomarkers. They argue tha

this technique may be superior becaus

the shapes and sizes of melanosomes ca

be altered during the mill ions of years i

which a fossil specimen lies buried and tha

entire specimens can be analyzed, obviatin

the need for analyses that alter or destrothe sample.

Research into fossil feathers is still at

nascent stage. In the last couple of years

feathers or feather-like structures hav

been found that represent the earliest stage

of dinosaur evolution. Now we are fill

ing in the colors. Feathers, perhaps even

brightly colored ones, may be primitive fo

dinosaurs [the discovery of the feathere

ornithischian Tianyulong(13) supports this

and pterosaurs and may even extend to th

base of the archosaur tree (14). We wil

hopefully improve on contemporary fancifudinosaur reconstructions by basing revision

directly on quantifiable results. The studie

by McKellar et al. and Wogelius et al. ar

way stations toward this goal.

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10.1126/science.1205748.

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(2005).

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The Life and Letters of Charles Darwin(John Murray,London, 1887), vol. 2, p. 296; (D. Appleton and Com-

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10.1126/science.121204

Primitive bird.An old illustration ofArchaeopteryx, generally considered to be the most primitive bird.

CREDIT:HEINRICHHARDER/FLORILEGIUS/ALAMY

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