EVOLUCIÓN

DEPARTAMENTO DE ECOLOGÍA, GENÉTICA Y EVOLUCIÓN

FACULTAD DE CIENCIAS EXACTAS Y NATURALES

UNIVERSIDAD DE BUENOS AIRES

2° CUATRIMESTRE 2020

GUÍA DE TRABAJOS PRÁCTICOS

MÓDULO 9

EVOLUCIÓN HUMANA

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TRABAJOS PRÁCTICOS DE EVOLUCIÓN HUMANA

Introducción general

El surgimiento de los homínidos puede ser analizado en el marco de los

mecanismos evolutivos aprendidos y discutidos durante la materia. Muchos aspectos de

la evolución del linaje humano pueden considerarse dentro del marco comprendido por

la selección natural ya que todas las especies, (los homínidos no son excepción) son

inherentemente variables y esa variabilidad afecta la capacidad de los individuos de

sobrevivir y reproducirse. Por otro lado, los recursos son limitados y por lo tanto, existe

una competencia entre los individuos por los mismos que resulta en la supervivencia y

reproducción diferencial. Sin embargo, existen otros factores que pueden afectar la

supervivencia y perpetuación de los seres vivos y además, existen otros niveles en los

que puede ocurrir la evolución biológica. Algunos de estos factores constituyen el

aspecto contingente de la evolución. Por ejemplo, cambios climáticos y geológicos a

gran escala pueden llevar a la extinción de poblaciones o especies enteras o crear

condiciones competitivas enteramente diferentes. Por otro lado, el carácter de los

cambios evolutivos ocurridos previamente en la historia de un linaje puede influenciar o

limitar el potencial de cambio de la especie. Este factor se conoce como la restricción

filogenética del proceso evolutivo.

A la hora de analizar la evolución humana hay que considerar otro elemento: la

potencialidad evolutiva adquirida con el desarrollo de un cerebro con capacidades no

expresadas previamente por otro organismo en la historia del planeta. Este evento trajo

aparejado el surgimiento de una evolución cultural sin antecedentes, un nuevo nivel

evolutivo muy ligado al desarrollo del lenguaje articulado. Como los mecanismos

darwinianos clásicos no son enteramente aplicables a éste nivel cultural, la cultura

humana no puede ser analizada solamente desde la perspectiva biológica, por regirse

por leyes propias, intrínsecamente distintas y cuya complejidad es irreducible a un plano

biológico puro. Podemos pensar a la evolución cultural como un sistema de cambio más

de tipo “lamarckiano” en donde todo lo aprendido en una generación puede ser

transmitido directamente a la siguiente generación por medio de la enseñanza y de la

escritura. Los caracteres adquiridos son transmitidos en la tecnología y la cultura. La

evolución lamarckiana es rápida y acumulativa, es por eso que en tiempos tan pequeños

– despreciables para los tiempos geológicos – nuestra tecnología nos permitió modificar

nuestro entorno de manera drástica. La evolución biológica continúa en Homo sapiens

pero lo hace a un ritmo nuevo, al compás de cambios impuestos (ahora además) por la

cultura.

Muchas características que presenta Homo sapiens lo diferencian inclusive de

las especies de primates más cercanas. La naturaleza de estos caracteres puede ser

cuantitativa o cualitativa y abarcan varios niveles de complejidad: desde las diferencias

genómicas, pasando por los patrones de desarrollo hasta llegar al comportamiento, las

estrategias reproductivas, la estructura social, el manejo de la información y la relación

de la especie con el resto de la biota. Entender como se produjeron esos cambios, como

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influyó el ambiente y cuales fueron las bases genéticas, de desarrollo o cognitivas que lo

permitieron son preguntas fundamentales a responder por la ciencia. Disciplinas tan

disímiles como la genómica comparada, la paleontología y la antropología social buscan

responder partes de este dilema. El desafío último consiste en la conciliación de toda la

información en una explicación completa de la evolución de nuestra especie.

A través de los trabajos prácticos del módulo de Evolución Humana se propone

organizar la discusión alrededor de tres aspectos fundamentales de la evolución de

nuestro linaje.

A- ¿Qué nos diferencia de otros primates? El orden Primate incluye una gran

diversidad de familias y especies (Fig. 1). Durante el primer TP se abordará el estudio

de ciertos aspectos característicos de otras especies de primates que constituyen el

grupo al cual pertenecemos: los monos antropoideos. Asimismo, se estudiará la base

biológica de nuestro parentesco con los chimpancés, especie viviente más cercanamente

emparentada con Homo sapiens. Este Se propone reflexionar sobre las características

que han surgido en ambos linajes luego de la divergencia.

B- Registro fósil de los homininos. Recientemente denominados homininos, este grupo

de primates fósiles bípedos que constituyen nuestro linaje se conoce a través de un

nutrido registro fósil. En el segundo TP de este bloque se propone analizar el patrón que

presenta dicho registro y discutir diferentes interpretaciones posibles. Se hará hincapié

en el estudio de aquellos caracteres que evolucionaron a lo largo del linaje y las

particularidades de los distintos grupos de homíninos, con el objetivo de inferir los

procesos evolutivos subyacentes a dicho patrón.

Figura 1: Árbol filogenético que muestra relaciones de parentesco probables entre

distintos grupos de primates, entre ellos, nuestra especie.

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SEMINARIO I

ORDEN PRIMATE:

NUESTROS PARIENTES CERCANOS

Durante este TP se propone reflexionar sobre la base biológica de nuestro parentesco

con el resto de las especies del Orden Primate. Se abordará el estudio de ciertos aspectos

característicos del grupo de especies de primates más cercanamente emparentadas con

el humano y que constituyen el grupo al cual pertenecemos: los monos antropoideos.

Se analizarán luego las características que han surgido en los linajes de chimpancés y

humanos actuales luego de la divergencia desde su ancestro común. Este estudio se

abordará a nivel morfológico a través de los extractos de Fleagle (1999) y mediante la

comparación de modelos 3D de cráneos de diferentes especies de Homínidos.

1. Lea atentamente el siguiente artículo y responda las preguntas que se detallan a

continuación:

Fleagle, J.G. 1999. Extractos del capítulo 7: “Apes and Humans” en Primate

Adaptation and Evolution. 2da. Ed. Elsevier.

1.1. ¿Cuántas especies de simios africanos viven en la actualidad? Explique el

modelo evolutivo propuesto para este grupo a partir de la figura 7.13.

1.2. ¿Cuáles son las principales similitudes y diferencias entre las especies de

hominoideos?

1.3. ¿Por qué considera que resulta dificultoso recuperar una filogenia robusta del

grupo de los monos antropoideos? ¿Qué significa “evolución en mosaico”?

2. Compare los cráneos de las siguientes especies de homínidos (Gorila gorila, Pan

troglodytes y Homo sapiens) y describa, de forma general, las diferencias que observa

en relación a las siguientes características:

- Robustez general

- Procesos óseos particulares: crestas óseas

- Forma de la arcada mandibular

- Dentición general, prognatismo alveolar

- Posición del foramen magnum

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- Prognatismo facial

- La posición de la frente (vertical o baja/huidiza).

- Presencia de mentón.

Ejemplares a comparar:

• Gorila gorila

Hembra adulta: https://sketchfab.com/3d-models/gorilla-gorilla-skull-52b02b8331004e9d9b39c96b75e2e3c5

Hembra juvenil: https://sketchfab.com/3d-models/gorilla-gorilla-skull-23c13920e86d4375a902f786733d5544

Macho adulto: https://sketchfab.com/3d-models/gorilla-skull-3302d69c29ba42f58f2c139cefbbf40e

• Pan troglodytes

Hembra adulta: https://sketchfab.com/3d-models/pan-troglodytes-skull-6eccede2c52e4edf990be8a46a8e9936

Macho adulto: https://sketchfab.com/3d-models/common-chimpanzee-casmam9806-d9487f673d664a3586bee21936e42936

• Pan panicus

Hembra adulta: https://sketchfab.com/3d-models/pan-paniscus-skull-b7ba07ddc00c4572bcd7a6a0f6faecec

• Homo sapiens

https://sketchfab.com/3d-models/the-anatomy-of-the-human-skull-baf6ac7b781a46218dca2b59dee58817

https://sketchfab.com/3d-models/homo-sapiens-sapiens-caaa46f2a58243ebb7927f27ff15173f

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SEMINARIO II

REGISTRO FÓSIL DE LOS HOMININOS

INTRODUCCIÓN GENERAL

Los orígenes de la humanidad es uno de los temas de la investigación científica que

despierta más pasiones y debates. Muchas hipótesis e ideas que eran aceptadas con entusiasmo en el

pasado han pasado al olvido y otras han surgido a medida que la evidencia se acumula y las técnicas

paleoantropológicas se refinan. Pero las interpretaciones de los fósiles distan mucho de ser

absolutamente objetivas. Las teorías sobre la evolución humana no sólo intentan enmarcar la

evidencia en un contexto lógico basado en las evidencias disponibles, sino que también nos brindan

información valiosa sobre el pensamiento de la época en que fueron formuladas.

Temas

- El registro fósil de los homininos. Patrones y criterios de interpretación.

- Herramientas para un análisis comparado: elementos de morfología cráneo-facial

- Historia de las teorías y miradas sobre la evolución humana.

- Las evidencias de evolución humana como patrón macroevolutivo.

- Lenguaje y pensamiento simbólico en relación a la evidencia paleontológica.

Tradicionalmente, las inferencias sobre el grado de parentesco existente entre el hombre

moderno y sus parientes cercanos (monos y simios) y fósiles (homininos) se han basado en el

análisis morfológico a nivel osteológico. Los huesos, en particular los huesos del cráneo, poseen

asociado un alto potencial de fosilización por lo que constituyen la mayoría de los restos del registro

fósil conocido sobre nuestro linaje.

Observe con detenimiento el siguiente esquema sintético de morfología cráneo-facial

comparativo e intente reconocer las estructuras óseas en los ejemplares que se hallarán a su

disposición en el TP.

Por otra parte, los restos líticos confeccionados y utilizados por los homininos también

poseen un alto potencial de conservación asociado. El patrón de distribución geográfico, su

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tipología y su variabilidad son rasgos característicos de este registro arqueológico, los cuales se

analizan en conjunto con los rasgos morfológicos presentes en los restos óseos. De manera muy

general, se reconocen en el linaje hominino una serie de “industrias líticas” o “complejos tecno-

culturales” diferentes asociadas a distintas especies. Si bien cada uno de estos complejos es

identificado a través de ciertos componentes diagnósticos, los mismos incluyen una serie variable

de utensilios en cuanto a la forma, el tamaño y la materia prima utilizada (McBrearty & Brooks,

2000). En este TP no analizaremos la variabilidad del registro arqueológico, ya que excede los

objetivos de la materia Evolución.

ACTIVIDADES A REALIZAR:

1- Su grupo dispondrá de información sobre distintos fósiles de homininos. A partir de la misma y

en forma grupal deberá analizar la morfología cráneo-facial teniendo en cuenta la información de la

figura.

Evidencia fósil de las diferentes especies:

- Género Australopithecus:

1) A. afarensis (3,8 a 3 millones de años):

https://sketchfab.com/3d-models/australopithecus-afarensis-25011rp13-skull-7fb668da6c244e0ba4e94bc244a65b20

2) A. africanus (3,2 a 2,4 millones de años):

https://sketchfab.com/3d-models/mrs-ples-a-africanus-87c74f78bc0e4ffb80ab955f54d581a6

- Género Paranthropus:

3) P. bosei (2,2 a 1,3 millones de años):

https://sketchfab.com/3d-models/australopithecus-boisei-4effe8a1a18140e3beccdb14f013538a

4) P. aethiopicus (2,7 a 2,2 millones de años):

https://sketchfab.com/3d-models/paranthropus-aethiopicus-skull-fossil-a27f740960f542a9b4732b8aeddf6831

- Género Homo:

5) H. habilis (2,4 a 1,5 millones de años):

https://sketchfab.com/3d-models/homo-habilis-cranium-sawyer-deak-fd935723406f423faedd3763ac2fd56a

6) H. rudolfensis (1,9 a 1,4 millones de años):

https://sketchfab.com/3d-models/homo-rudolfensis-cranium-knm-er-1470-d01174f8d7aa42e48b56a83ee78a7de3

7) H. erectus (1,9 a 0,5 millones de años):

https://sketchfab.com/3d-models/homo-erectus-25011rp84-1-cranium-196af30c673540b2bee7b51c9a2a50a8

https://sketchfab.com/3d-models/homo-erectus-peking-man-1979rp44-dbedc202dbb14f32bf948aec752ab0cb

8) H. ergaster (1,8 a 0,6 millones de años):

https://sketchfab.com/3d-models/homo-ergaster-25011rp31-cranium-5581a3f01ae8493abba6be522bc1ea05

9) H. neanderthalensis (230000 a 35000 años): https://sketchfab.com/3d-models/homo-neanderthalensis-cranium-sawyer-maley-3e5cbb12854d4716a64415595c7896cb

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10) H. floresiensis (190000 a 50000 años):

https://sketchfab.com/3d-models/homo-floresiensis-cranium-49390fffcfda49ca8694f6c42c6364be

11) H. sapiens (180000 años al presente):

https://sketchfab.com/3d-models/the-anatomy-of-the-human-skull-baf6ac7b781a46218dca2b59dee58817

2- Leer el siguiente texto y responder las preguntas asociadas:

Tattersall, I. 2005. Once we were not alone. Scientific American 25-32.

a. ¿Qué planteaba la hipótesis de especie única? ¿Qué factores contribuían al mantenimiento

de esta línea de pensamiento?

b. ¿Cómo afectaron los descubrimientos efectuados a partir de 1970 a la concepción que se

tenía del modo de evolución del linaje humano? ¿Cómo es ahora el panorama atribuible a la

diversidad de homínidos hace 1.8 millones de años?

c. ¿Cuál es la edad de los fósiles más antiguos pertenecientes al género Australopithecus? ¿Y

del género Homo?

d. ¿Qué rol se le atribuye a Neandertal (y a las poblaciones asiáticas de H. erectus) en una

visión lineal de la evolución? ¿Qué propone la teoría arbustiva?

e. ¿Qué propone Tattersall acerca de los neandertales? ¿Qué diferencias se observan en la

coexistencia ocurrida entre H. sapiens y H. neanderthalensis entre Medio Oriente y Europa?

¿Qué propone el autor acerca del resultado de esas interacciones?

f. En octubre de 2004 se describió a una nueva especie de homínido: Homo floresiensis. Esta

especie existió hasta hace 13.000 años en una isla del sudeste asiático. ¿Cómo repercute este

descubrimiento en el modelo sostenido por Tattersal?

g. ¿Cuál habría sido según Tattersall la característica clave del éxito de H. sapiens y cómo se

habría establecido? ¿Cómo se aplican en este caso los conceptos de exaptación y de

emergencia?

h. ¿Cómo interpreta Tattersall el hecho que los patrones de comportamiento moderno de los

humanos, que denotan la existencia de pensamiento simbólico, se establecieron mucho

tiempo después que las características anatómicas típicas de H. sapiens?

BIBLIOGRAFÍA COMPLEMENTARIA

• McBrearty, S.; Brooks, A.S. 2000. The revolution that wasn’t: a new interpretation of the origin of

modern human behavior. J Hum Evol 39:453-563.

• d'Errico, F. 2003. The invisible frontier. A multiple species model for the origin of behavioral

modernity. Evolutionary Anthropology 12 (4):188-202.

• Hardt, T. Handbook of Paleoanthropology: Vol I: Principles, Methods and Approaches Vol II:

Primate Evolution and Human Origins Vol III: Phylogeny of Hominids. Springer Science &

Business Media, 2007. (libro disponible en el DRIVE de la materia)

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ONCE we

SHARING A SINGLE LANDSCAPE, four kinds of hominids lived about 1.8 million years ago in what is now part of northern Kenya.Although paleoanthropologists have no idea how—or if—these different species interacted, they do know that Paranthropus boisei, Homo rudolfensis, H. habilis and H. ergaster foraged in the same area around Lake Turkana.

TODAY WE TAKE FOR GRANTED THAT HOMO SAPIENS

FOUR MILLION YEARS MANY HOMINID SPECIES

EMERGENCEoriginally published in 2003

COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. 15

were not alone

IS THE ONLY HOMINID ON EARTH. YET FOR AT LEAST

SHARED THE PLANET. WHAT MAKES US DIFFERENT?

By Ian Tattersall • Paintings by Jay H. Matternes

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for the past 25,000 years or so, free andclear of competition from other mem-bers of the hominid family. This periodhas evidently been long enough for us tohave developed a profound feeling thatbeing alone in the world is an entirelynatural and appropriate state of affairs.

So natural and appropriate, indeed,that during the 1950s and 1960s aschool of thought emerged that claimed,in essence, that only one species of hom-inid could have existed at a time becausethere was simply no ecological space onthe planet for more than one culture-bearing species. The “single-species hy-pothesis” was never very convincing—

even in terms of the rather sparse homi-nid fossil record of 40 years ago. But theimplicit scenario of the slow, single-minded transformation of the bent andbenighted ancestral hominid into thegraceful and gifted modern H. sapiensproved powerfully seductive—as fablesof frogs becoming princes always are.

So seductive that it was only in thelate 1970s, following the discovery of in-controvertible fossil evidence that hom-inid species coexisted some 1.8 million

years ago in what is now northern Kenya,that the single-species hypothesis wasabandoned. Yet even then, paleoanthro-pologists continued to cleave to a ratherminimalist interpretation of the fossilrecord. Their tendency was to downplaythe number of species and to group to-gether distinctively different fossils un-der single, uninformative epithets suchas “archaic Homo sapiens.” As a result,they tended to lose sight of the fact thatmany kinds of hominids had regularlycontrived to coexist.

Although the minimalist tendencypersists, recent discoveries and fossilreappraisals make clear that the biolog-ical history of hominids resembles thatof most other successful animal families.It is marked by diversity rather than bylinear progression. Despite this rich his-tory—during which hominid species de-veloped and lived together and compet-ed and rose and fell—H. sapiens ulti-mately emerged as the sole hominid. Thereasons for this are generally unknow-able, but different interactions betweenthe last coexisting hominids—H. sapiensand H. neanderthalensis—in two dis-

tinct geographical regions offer some in-triguing insights.

A Suite of Species FROM THE BEGINNING, almost fromthe very moment the earliest hominidbiped—the first “australopith”—madeits initial hesitant steps away from theforest depths, we have evidence for hom-inid diversity. The oldest-known poten-tial hominid is Sahelanthropus tchaden-sis, represented by a cranium from thecentral-western country of Chad [see il-lustration on page 26]. Better known isAustralopithecus anamensis, from sitesin northern Kenya that are about 4.2million years old.

A. anamensis looks reassuringly simi-lar to the 3.8- to 3.0-million-year-oldAustralopithecus afarensis, a small-brained, big-faced bipedal species towhich the famous “Lucy” belonged.Many remnants of A. afarensis havebeen found in various eastern Africansites, but some researchers have suggest-ed that the mass of fossils described as A.afarensis may contain more than onespecies, and it is only a matter of time

Homo sapiens has had the earth to itself

HOMO RUDOLFENSISwas a relativelylarge-brainedhominid, typified bythe famous KNM-ER1470 cranium. Itsskull was distinctfrom the apparentlysmaller-brained H.habilis, but its bodyproportions areeffectively unknown.

PARANTHROPUS BOISEIhad massive jaws,equipped with hugegrinding teeth for apresumed vegetariandiet. Its skull isaccordingly stronglybuilt, but it is notknown if in body size itwas significantly largerthan the “gracile”australopiths.

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until the subject is raised again. In anyevent, A. afarensis was not alone inAfrica. A distinctive jaw, from an aus-tralopith named A. bahrelghazali, wasfound in 1995 in Chad. It is probablybetween 3.5 and 3.0 million years oldand is thus roughly coeval with Lucy, asis the recently named new form Kenyan-thropus platyops.

In southern Africa, scientists reportedevidence in 1999 of another primitivebipedal hominid species. As yet un-named and undescribed, this distinctiveform is 3.3 million years old. At aboutthree million years ago, the same regionbegins to yield fossils of A. africanus, thefirst australopith to be discovered (in1924). This species may have persisteduntil not much more than two millionyears ago. A 2.5-million-year-old speciesfrom Ethiopia, named Australopithecusgarhi in 1999, is claimed to fall in an in-termediate position between A. afaren-sis, on the one hand, and a larger groupthat includes more recent australopithsand Homo, on the other. Almost exact-ly the same age is the first representativeof the “robust” group of australopiths,Paranthropus aethiopicus. This earlyform is best known from the 2.5-mil-lion-year-old “Black Skull” of northernKenya, and in the period between about2 and 1.4 million years ago the robustswere represented all over eastern Africaby the familiar P. boisei. In South Africa,during the period around 1.6 millionyears ago, the robusts included the dis-

tinctive P. robustus and possibly a close-ly related second species, P. crassidens.

I apologize for inflicting this long listof names on readers, but in fact it actu-ally underestimates the number of aus-tralopith species that existed. What ismore, scientists don’t know how longeach of these creatures lasted. Neverthe-less, even if average species longevitywas only a few hundred thousand years,it is clear that from the very beginningthe continent of Africa was at least pe-riodically—and most likely continual-ly—host to multiple kinds of hominids.

The appearance of the genus Homodid nothing to perturb this pattern. The2.5- to 1.8-million-year-old fossils fromeastern and southern Africa that an-nounce the earliest appearance of Homoare an oddly assorted lot and probably alot more diverse than their conventionalassignment to the two species H. habilisand H. rudolfensis indicates. Still, atKenya’s East Turkana, in the period be-tween 1.9 and 1.8 million years ago,these two species were joined not onlyby the ubiquitous P. boisei but by H. er-gaster, the first hominid of essentiallymodern body form. Here, then, is evi-dence for four hominid species sharingnot just the same continent but the samelandscape [see illustration on previouspage and below].

The first exodus of hominids fromAfrica, presumably in the form of H. er-gaster or a close relative, opened a vastprospect for further diversification. One

could wish for a better record of thismovement, and particularly of its dat-ing, but there are indications that hom-inids of some kind had reached Chinaand Java by about 1.8 million years ago.A lower jaw that may be about the sameage from Dmanisi in ex-Soviet Georgiais different from anything else yet found[see “Out of Africa Again ... and Again?”by Ian Tattersall; SCIENTIFIC AMERICAN,April 1997]. By the million-year markH. erectus was established in both Javaand China, and it is possible that a morerobust hominid species was present inJava as well. At the other end of theEurasian continent, the oldest-knownEuropean hominid fragments—fromabout 800,000 years ago—are highlydistinctive and have been dubbed H. an-tecessor by their Spanish discoverers.

About 600,000 years ago, in Africa,we begin to pick up evidence for H. hei-delbergensis, a species also seen at sitesin Europe—and possibly China—be-tween 500,000 to 200,000 years ago. Aswe learn more about H. heidelbergensis,we are likely to find that more than onespecies is actually represented in thisgroup of fossils. In Europe, H. heidel-bergensis or a relative gave rise to an en-demic group of hominids whose best-known representative was H. nean-derthalensis, a European and westernAsian species that flourished betweenabout 200,000 and 30,000 years ago.The sparse record from Africa suggeststhat at this time independent develop-

HOMO HABILIS(“handy man”) wasso named because itwas thought to be themaker of the 1.8-million-year-oldstone toolsdiscovered at OlduvaiGorge in Tanzania.This hominidfashioned sharpflakes by banging one rock cobbleagainst another.

HOMO ERGASTER,sometimes called “AfricanH. erectus,” had a high,rounded cranium and askeleton broadly similarto that of modernhumans. Although H.ergaster clearly ate meat,its chewing teeth arerelatively small. The bestspecimen of this hominidis that of an adolescentfrom about 1.6 millionyears ago known asTurkana boy.

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ments were taking place there, too—in-cluding the emergence of H. sapiens.And in Java, possible H. erectus fossilsfrom Ngandong were dated to around40,000 years ago, implying that this areahad its own indigenous hominid evolu-tionary history for perhaps millions ofyears as well.

The picture of hominid evolution justsketched is a far cry from the “Australo-pithecus africanus begat Homo erectusbegat Homo sapiens” scenario that pre-vailed 40 years ago—and it is, of course,based to a great extent on fossils thathave been discovered since that time.Yet the dead hand of linear thinking stilllies heavily on paleoanthropology, andeven today quite a few of my colleagueswould argue that this scenario overesti-mates diversity. There are various waysof simplifying the picture, most of them

involving the cop-out of stuffing all vari-ants of Homo of the past half a millionor even two million years into the speciesH. sapiens.

My own view, in contrast, is that the20 or so hominid species invoked (if notnamed) above represent a minimum es-timate. Not only is the human fossilrecord as we know it full of largely un-acknowledged morphological indica-tions of diversity, but it would be rash toclaim that every hominid species thatever existed is represented in one fossilcollection or another. And even if onlythe latter is true, it is still clear that thestory of human evolution has not beenone of a lone hero’s linear struggle.

Instead it has been the story of na-ture’s tinkering: of repeated evolution-ary experiments. Our biological historyhas been one of sporadic events rather

than gradual accretions. Over the pastfive million years, new hominid specieshave regularly emerged, competed, co-existed, colonized new environmentsand succeeded—or failed. We have onlythe dimmest of perceptions of how thisdramatic history of innovation and in-teraction unfolded, but it is already evi-dent that our species, far from being thepinnacle of the hominid evolutionarytree, is simply one more of its many ter-minal twigs.

The Roots of Our Solitude ALTHOUGH THIS is all true, H. sapi-ens embodies something that is undeni-ably unusual and is neatly captured bythe fact that we are alone in the worldtoday. Whatever that something is, it isrelated to how we interact with the ex-ternal world: it is behavioral, which

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TUC D’AUDOUBERT CAVE in France was entered sometime between perhaps11,000 and 13,000 years ago by H. sapiens, also called Cro Magnons, whosculpted small clay bison in a recess almost a mile underground.

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means that we have to look to our ar-chaeological record to find evidence ofit. This record begins some 2.5 millionyears ago with the production of the firstrecognizable stone tools: simple sharpflakes chipped from parent “cores.” Wedon’t know exactly who the inventorwas, but chances are that he or she wassomething we might call an australopith.

This landmark innovation represent-ed a major cognitive leap and had pro-found long-term consequences for hom-inids. It also inaugurated a pattern ofhighly intermittent technological change.It was a full million years before the nextsignificant technological innovationcame along: the creation about 1.5 mil-lion years ago, probably by H. ergaster,of the hand ax. These symmetrical im-plements, shaped from large stone cores,were the first tools to conform to a “men-tal template” that existed in the tool-maker’s mind. This template remainedessentially unchanged for another mil-lion years or more, until the invention of“prepared-core” tools by H. heidelber-gensis or a relative. Here a stone core waselaborately shaped in such a way that asingle blow would detach what was aneffectively finished implement.

Among the most accomplished practi-tioners of prepared-core technologywere the large-brained, big-faced andlow-skulled Neandertals, who occupiedEurope and western Asia until about30,000 years ago. Because they left anexcellent record of themselves and wereabruptly replaced by modern humans

who did the same, the Neandertals fur-nish us with a particularly instructiveyardstick by which to judge our ownuniqueness. The stoneworking skills ofthe Neandertals were impressive, ifsomewhat stereotyped, but they rarely ifever made tools from other preservablematerials. And many archaeologistsquestion the sophistication of their hunt-ing skills.

Further, despite misleading early ac-counts of bizarre Neandertal “bearcults” and other rituals, no substantialevidence has been found for symbolicbehaviors among these hominids or forthe production of symbolic objects—cer-tainly not before contact had been madewith modern humans. Even the occa-sional Neandertal practice of buryingthe dead may have been simply a way ofdiscouraging hyenas from making in-cursions into their living spaces or havea similar mundane explanation. Thisview arises because Neandertal burials

lack the “grave goods” that would attestto ritual and belief in an afterlife. TheNeandertals, in other words, though ad-mirable in many ways and for a longtime successful in the difficult circum-stances of the late ice ages, lacked thespark of creativity that, in the end, dis-tinguished H. sapiens.

Although the source of H. sapiens asa physical entity is obscure, most evi-dence points to an African origin perhapsbetween 150,000 and 200,000 yearsago. Modern behavior patterns did notemerge until much later. The best evi-dence comes from Israel and its sur-rounding environs, where Neandertalslived about 200,000 years ago or per-haps even earlier. By about 100,000years ago, they had been joined byanatomically modern H. sapiens, andthe remarkable thing is that the toolsand sites the two hominid species left be-hind are essentially identical. As far ascan be told, these two hominids behaved

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IAN TATTERSALL and JAY H. MATTERNES have worked together since the early 1990s, no-tably on the Hall of Human Biology and Evolution at the American Museum of Natural His-tory in New York City and at the Gunma Museum of Natural History in Tomioka, Japan (wherethe Tuc d’Audoubert mural on the opposite page is installed). Tattersall was born in Englandand raised in East Africa. He is a curator in the department of anthropology at the Ameri-can Museum of Natural History. His books include Becoming Human: Evolution and HumanUniqueness (Harvest Books, 1999) and The Last Neanderthal: The Rise, Success, and Mys-terious Extinction of Our Closest Human Relatives (Westview Press, 1999, revised).

Matternes is an artist and sculptor who has for more than 40 years specialized in fos-sil primates and hominids. In addition to his museum murals, he is well known for his illus-trations for books, periodicals and television, including Time/Life Books and National Geo-graphic. The research for his depictions has taken him to many parts of the U.S., Canada,Mexico, France, Colombia and Africa.

THE

AU

THO

R A

ND

TH

E A

RTI

ST

HOMINIDS of modern body form mostlikely emerged in Africa around 150,000years ago and coexisted with otherhominids for a time before emerging asthe only species of our family. Until about 30,000 years ago, they overlappedwith H. neanderthalensis (left) in Europeand in the Levant, and they may havebeen contemporaneous with theH. erectus (right) then living in Java.

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PAT

RIC

IA J

. W

YNN

E (

dra

win

gs)

in similar ways despite their anatomicaldifferences. And as long as they did so,they somehow contrived to share theLevantine environment.

The situation in Europe could hardlybe more different. The earliest H. sapi-ens sites there date from only about40,000 years ago, and just 10,000 or soyears later the formerly ubiquitous Ne-andertals were gone. Significantly, theH. sapiens who invaded Europe broughtwith them abundant evidence of a fullyformed and unprecedented modern sen-sibility. Not only did they possess a new“Upper Paleolithic” stoneworking tech-nology based on the production of mul-tiple long, thin blades from cylindricalcores, but they made tools from boneand antler, with an exquisite sensitivityto the properties of these materials.

Even more significant, they broughtwith them art, in the form of carvings,engravings and spectacular cave paint-ings; they kept records on bone andstone plaques; they made music on windinstruments; they crafted intricate per-sonal adornments; they afforded someof their dead elaborate burials withgrave goods (hinting at social stratifica-tion in addition to belief in an afterlife,for not all burials were equally fancy);and their living sites were highly orga-nized, with evidence of sophisticatedhunting and fishing. The pattern of in-termittent technological innovation wasgone, replaced by constant refinement.Clearly, these people were us.

Competing ScenariosIN ALL THESE WAYS, early Upper Pa-leolithic people contrasted dramaticallywith the Neandertals. Some Neandertalsin Europe seem to have picked up newways of doing things from the arrivingH. sapiens, but we have no direct cluesas to the nature of the interaction be-tween the two species. In light of the Ne-andertals’ rapid disappearance and ofthe appalling subsequent record of H.sapiens, though, we can reasonably sur-mise that such interactions were rarelyhappy for the former. Certainly the re-peated pattern found at archaeologicalsites is one of short-term replacement,and there is no convincing biological ev-

SPECULATIVE FAMILY TREE shows the variety of hominidspecies that have populated the planet—some identified byonly a fragment, others known to exist for a specific timeperiod (solid lines). The emergence of H. sapiens has notbeen a single, linear transformation of one species intoanother but rather a meandering, multifaceted evolution.

H. sapiens (Worldwide)

H. heidelbergensis (throughout Old World)

H. neanderthalensis (Europe and Western Asia)

H. antecessor(Spain)

0

1

2

3

4

5

6

Au. africanus(South Africa)

Orrorin tugenensis(Kenya)

Ardipithecus ramidus(Ethiopia)

Sahelanthropus tchadensis

(Chad)

Australopithecusanamensis(Kenya)

Au. afarensis (Ethiopia and

Tanzania)

Au. garhi(Ethiopia)

Paranthropusaethiopicus

(Eastern Africa)

Mill

ions

ofY

ears

Ago

Homo ergaster(Eastern Africa)

P. robustus(South Africa)

P. boisei(Eastern Africa)

H. erectus (Eastern Asia)

H. habilis(Sub-Saharan Africa)

Au. bahrelghazali(Chad)

Kenyanthropusplatyops(Kenya)

K. rudolfensis(Eastern Africa)

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idence of any intermixing of peoples inEurope.

In the Levant, the coexistence ceased—

after about 60,000 years or so—at rightabout the time that Upper Paleolithic–like tools began to appear. About 40,000years ago the Neandertals of the Levantyielded to a presumably culturally richH. sapiens, just as their European coun-terparts had.

The key to the difference between theEuropean and the Levantine scenarioslies, most probably, in the emergence ofmodern cognition—which, it is reason-able to assume, is equivalent to the ad-vent of symbolic thought. Business hadcontinued more or less as usual rightthrough the appearance of modern bonestructure, and only later, with the ac-quisition of fully modern behavior pat-terns, did H. sapiens become complete-ly intolerant of competition from itsnearest—and, evidently, not its dearest—co-inhabitors.

To understand how this change in sen-sibility occurred, we have to recall cer-tain things about the evolutionary pro-cess. First, as in this case, all innovationsmust necessarily arise within preexistingspecies—for where else can they do so?Second, many novelties arise as “exap-tations,” features acquired in one con-text before (often long before) being co-opted in a different one. For example,hominids possessed essentially modernvocal tracts for hundreds of thousandsof years before the behavioral recordgives us any reason to believe that theyemployed the articulate speech that thepeculiar form of this tract permits.

And finally, it is important to bear inmind the phenomenon of emergence—

the notion that a chance coincidencegives rise to something totally unexpect-ed. The classic scientific example in thisregard is water, whose properties arewholly unpredicted by those of hydro-gen and oxygen atoms alone. If we com-bine these various observations, we can

see that, profound as the consequencesof achieving symbolic thought may havebeen, the process whereby it came aboutwas unexceptional.

We have no idea at present how themodern human brain converts a mass ofelectrical and chemical discharges intowhat we experience as consciousness.We do know, however, that somehowour lineage passed to symbolic thoughtfrom some nonsymbolic precursor state.The only plausible possibility is thatwith the arrival of anatomically modernH. sapiens, existing exaptations werefortuitously linked by a relatively minorgenetic innovation to create an unprece-dented potential.

Yet even in principle this deduced sce-nario cannot be the full story, becauseanatomically modern humans behavedarchaically for a long time before adopt-ing modern behaviors. That discrepan-cy may be the result of the late appear-ance of some key hardwired innovationnot reflected in the skeleton, which is allthat fossilizes. But this seems unlikely,because it would have necessitated awholesale Old World–wide replacementof hominid populations in a very shorttime, something for which there is no evidence.

It is much more likely that the modernhuman capacity was born at—or closeto—the origin of H. sapiens, as an abili-ty that lay fallow until it was activatedby a cultural stimulus of some kind. If

sufficiently advantageous, this behav-ioral novelty could then have spreadrapidly by cultural contact among pop-ulations that already had the potential toacquire it. No population replacementwould have been necessary to spread thecapability worldwide.

It is impossible to be sure what this in-novation might have been, but the bestcurrent bet is that it was the invention oflanguage. For language is not simply themedium by which we express our ideasand experiences to one another. Ratherit is fundamental to the thought processitself. It involves categorizing and nam-ing objects and sensations in the outerand inner worlds and making associa-tions between resulting mental symbols.It is, in effect, impossible for us to con-ceive of thought (as we are familiar withit) in the absence of language, and it isthe ability to form mental symbols thatis the fount of our creativity. Only whenwe are able to create such symbols canwe recombine them and ask such ques-tions as “What if...?”

We do not know exactly how lan-guage might have emerged in one localpopulation of H. sapiens, although lin-guists have speculated widely. But we doknow that a creature armed with sym-bolic skills is a formidable competitor—

and not necessarily an entirely rationalone, as the rest of the living world, in-cluding H. neanderthalensis, has discov-ered to its cost.

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Dark Caves, Bright Visions: Life in Ice Age Europe. Randall White. W. W. Norton/American Museumof Natural History, 1986. Language and Species. Reprint edition. Derek Bickerton. University of Chicago Press, 1992.The Fossil Trail: How We Know What We Think We Know about Human Evolution. Ian Tattersall.Oxford University Press, 1995.Getting Here: The Story of Human Evolution. Updated edition. William Howells. Compass Press,1997.African Exodus: The Origins of Modern Humanity. Reprint edition. Christopher Stringer and Robin McKie. Henry Holt, 1998.The Origin and Diversification of Language. Edited by Nina G. Jablonski and Leslie C. Aiello.University of California Press, 1998.The Last Neanderthal: The Rise, Success and Mysterious Extinction of Our Closest HumanRelatives. Revised edition. Ian Tattersall. Westview Press, 1999.

M O R E T O E X P L O R E

The pattern of intermittent technological innovation was gone, replaced by constant refinement.

Clearly, these people were us.

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