Was the First Craniate on the Road to Cognition?
A Modern Craniate’s Perspective
Fredric J. Heeren
Abstract Chinese
fossil discoveries of the earliest known craniates (from the early
Cambrian period) have led scientists to question whether the evolution
of human-level cognition is a rare occurrence in the universe. The
earliest chordate is now best represented by a well-documented metazoan
called Haikouella lanceolata. Possessing a relatively large brain, this animal appears
to demonstrate that the brain and endoskeleton did not evolve together,
as had been assumed, but rather that the brain appeared long before
full endoskeletization. The paleontologist who describes the animal
further notes a “top-down” pattern in the appearances of
new forms in the fossil record. Researchers find such observations
relevant to the question: Was the evolution of human-level cognition in
some sense inevitable, or was it an accident dependent upon historical
contingencies? The new evidence for early craniates lends support to
the view that human- level cognition may be part of a developmental
package, but historical contingencies pose serious problems for a
strictly law-like explanation.
Key words Contingency,
convergence, developmentalism, directionality, internalism, saltation,
top-down evolution.
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One might propose three mutually exclusive
hypotheses to explain the existence of human- level cognition 2
observed on Earth:
1. Human-level cognition occurs as a fluke in the universe; on
Earth it may be explained as an incidental result of highly contingent
events. 3
2. Human-level cognition is a common occurrence throughout the
universe; its existence on Earth may be explained as a result of highly
constrained or law-like processes.
3. Human-level cognition on Earth can be explained
as neither fluke
nor the result of law-like processes; 4 the frequency of its occurrence
in the universe may not be predictable.
An investigator who knew that hypothesis 1 or 2
was correct would be able to comment knowledgeably upon the probability
of human-level cognition existing on any planet where signs of life
were observed. To estimate the probability of human-level cognition on
such a planet, a scientist might only need to know the length of time
that the planet had sustained conditions for life. Ideally, an
investigator attempting to choose between the above hypotheses should
observe life at several stages on many habitable planets before
claiming confidence in his selection. Of course, the best an Earthbound
scientist can do at present is to search for helpful clues on the one
planet where we know that life exists; and here at least Earth’s
crust does provide helpful access to pertinent glimpses of past life
stages. Of greatest relevance for any such researcher is the fossil
record’s snapshot in which the body plan 5 first appears that
would eventually house human-level cognition.
To join such an investigation, we would like to
explore the following questions: When does this body plan appear
relative to other body plans? Does this body plan have an assured, or
at least a likely, survival to permit time for the evolution of
cognition—or is the survival of its future lineage dependent upon
highly contingent events? Do other body plans appear that would also
seem to make good candidates for the support of human-level cognition?
How predictable is the appearance of a suitable, large brain-carrying
body plan? For that matter, upon what basis can we predict body plans
at all, or changes in any form of life? Does evolution proceed in a bottom-up
fashion where small changes accrue into large ones aided by no higher
law than natural selection, or are there top-down forces at work,
driving organisms more forcefully toward obligatory forms?
Examining these questions takes us back to the
early Cambrian period, to the time, 530 million years ago, when
virtually all the major animal groups, called phyla, first appear in
the fossil record. In December 1999, the journal Nature announced the discovery
there of what appears to be the remains of our earliest chordate
ancestor (CHEN /HUANG /LI 1999a). Found in southern China’s
Yunnan Province near Kunming, the stiffspined, paper clip-size animal
named Haikouella lanceolata pushes back the evolution of our own phylum, Chordata , to the very start
of modern metazoan life. Until its discovery, paleontologists had begun
to despair of ever finding undisputed evidence that our own
“advanced” phylum existed as early as the main burst of the
Cambrian “explosion” of new body plans. The plentiful
fossil evidence for Haikouella— 305 specimens, many in excellent condition—finally
confirms what many had suspected in recent years: Our own phylum
arrived on the scene along with most of the others, during the
surprisingly quick radiation of new major animal groups that
characterizes the early Cambrian period.
The discovery demonstrates that our chordate
ancestors had to fight their way through the rough and tumble Cambrian
seas, in a period when many animal groups that entered did not survive
to the end. Most families—and even many whole body
plans—disappeared (GOULD 1989, pp47n; WARD 2000, p184). As we
detail below, many paleontologists further classify Haikouella as a craniate. Any
argument for the inevitability of craniate survival could be made more
confidently if this body plan had arisen after
the Cambrian, since the Cambrian represents
the only period in which the number of animal phyla actually decreased
(Dobzhansky et al. 19 77, pp. 422–423; SOLÉ /GOODWIN 2000, pp.
249– 251). Thus, as we will see, the appearance of the craniate
body plan in the early Cambrian has paradoxical implications for a
heady future; it can be taken either as evidence for a head start on
the pathway to intelligence, or evidence of heightened probability that
this lineage would be cut off—at its head. Haikouella simultaneously creates
support for two competing inferences: the evolution of cognition as a
highly constrained, or as a highly contingent, process.
Current Controversies
The animal’s discovery thus raises questions
at the heart of current controversies in evolution research. One of the
broadest ways to characterize the competing positions is as a
disagreement between externalists and internalists, i.e., those who
treat external selection as virtually the sole creative force in
evolution versus those who emphasize the importance of internal
constraints. Related issues tend to pit developmentalists against
neo-DARWINISTS , formalists against functionalists, punctuationalists
against gradualists, and top-down theorists against bottomup theorists,
with the first party in each pair siding with the internalists. 6
Perhaps the most profound movement in the field in
recent decades has been the advancement of developmental biology (which
studies the way genes control the growth of individual living
organisms) as a key to evolutionary biology. Since the mid-1990s, the
marriage between the two disciplines has become known as evolutionary
developmental biology, or simply “evo-devo”. Evo-devo
explores how changes in ontogeny (the development of individual
organisms, from fertilized egg to maturity) are related to the
emergence of new phenotypes over successive generations. The goal is to
use knowledge of how genes control the development of embryonic
structures to learn how these same genes were involved in the first
appearance of such structures in past epochs. Early contributors to the
field include DE BEER (1930), SCHMALHAUSEN (1949), WADDINGTON (1957),
RAFF /KAUFFMAN (1983), ARTHUR (1988), and HALL (1992).
Developmentalists believe that too much credit has been given to the
power of natural selection.
Wallace ARTHUR finds it “strange …
that mainstream neo-DARWINIAN theory has come to regard natural
selection as the primary mechanism causing evolutionary change.
Selection is a destructive force, which acts only to eliminate”
(ARTHUR 1997, p241). Though trained as a neo-DARWINist, ARTHUR
acknowledges that selection “does not create the new type in the
first place”, adding: “We have come to accept a theory of
evolution that explains the origin and diversification of exquisitely
engineered organisms on the basis of the selective destruction of
genetic/developmental variants whose initial production has been
treated, for the most part, as a ‘black box’” (Ibid).
When it comes to the origin of body plans, according to ARTHUR ,
explaining it “purely in selective terms, without reference to
the underlying genetic architecture will ultimately fail. Hence the
need for the new discipline of evolutionary developmental
biology” (p. 291).
Harvard zoologist Stephen Jay GOULD , who died in
2002, noted that DARWIN himself had not believed that natural selection
was the exclusive means of evolutionary modification. Nevertheless, he
said, certain “ultra-DARWINISTS ” were trying to
“out-DARWIN DARWIN ” by claiming “that natural
selection regulates everything of any importance in evolution”
(GOULD 1997). For their part, neo-DARWINISTS accused G OULD of
wandering too far from the reductionism 7 so necessary to
science’s success. Daniel DENNETT even suggested that GOULD must
have had a “hidden agenda” to sneak purpose back into
biology (DENNETT 1996). The accusation was patently false; GOULD never
wrote with any such intent; in fact, he went to great lengths to create
new terms to replace any that smacked of teleology.
“Preadaptations” became “exaptations” precisely
to avoid the teleological overtones; “saltation” and
“laws of form” became “facet-flipping” (GOULD
2002, pp342– 351), helping to assure that the element of chance
overshadowed any Platonic connotations.
In his last monograph ,
GOULD wrote: “I argue that
‘internalism’ poses two separate challenges to pure
DARWINIAN functionalism: saltational change arising from internal
forces of mutability, and inherent directionality of variation…
Most internalists … emphasize the second theme of channels and
preferred directionality of variation” (2002, p445). One of the
traits that made GOULD stand out from other internalists was his
emphasis on chance. He seemed intent to exonerate himself from any
charge of advocating teleology by assuring us that, though constraints
have their important place, stochastic events have the final word. Our
own existence is attributable to a “golden happenstance” in
the Cambrian explosion (p. 1159), the start of an unlikely course that
continued with the repeated overcoming of odds to produce humans as
“an ultimate in oddball rarity” (1996–1997). If the
developmental patterns of bilaterians appear to have become fixed into
“limited and excellent, perhaps even optimal, designs”, it
is only because they represent just one possible solution among
numerous entirely plausible alternatives of strikingly different form,
each yielding a subsequent history of life entirely different from the
outcome actually experienced on earth” (2002, p1159). Yet all who
support development’s importance to evolution (including GOULD ,
who used terms like “directionality” and “congealed
designs”) must support the concept of directionality, however
they explain it.
ARTHUR uses the term “directionality”
to describe the observation of large-scale evolutionary changes
preceding smaller modifications, “big ‘experiments”
giving way to progressively more restricted modifications”
(ARTHUR 1997, p207), i.e., the top-down concept that we will explore
below. He writes: “This idea of directionality—which is
absent from conventional neo-DARWINIAN theory —is important in
Evolutionary Developmental Biology” (Ibid).
Against this background, we can now return to the
findings about the earliest known craniate, to note the support they
yield for this article’s first two hypothetical options:
human-level cognition as either fluke or common occurrence. We can also
compare the support they afford straightforward neo- DARWINISM versus
developmentalism.
A listing of Haikouella
-related supporting evidence for
developmental constraints—and the ubiquity of human-level
cognition in the universe —could read as follows:
1) early priority of cephalization over
endoskeletization,
2) constraints/convergence/channeling, 8
3) hierarchical phylogenies,
4) saltation,
5) top-down pattern in the fossil record, and
6) the principle of mediocrity.
Supporting arguments for contingency—and the
rarity of human-level cognition in the universe— would include:
1) The appearance of craniates in the early
Cambrian (the one period where the number of phyla decreases) greatly
increases the probability that this lineage would become extinct;
2) Non-chordate phyla can be shown to be incapable
of developing human-level cognition; and
3) Human-level cognition has evolved only once on
this planet.
As we recount the findings reported by Haikouella’s investigators,
we will examine each of these in turn.
Settling the Matter of Chordates in the Cambrian
Before Jun-Yuan CHEN ’s discovery of Haikouella (CHEN /HUANG /LI
1999a; CHEN /HUANG /LI 1999b) (Figures 1 and 2), it was beginning to
look doubtful that Cambrian chordates would ever become firmly
established. Each new interpretation of the mostly poorly preserved
would-be-chordate specimens was hotly disputed.
Biologists belonging to the contingency camp, as
opposed to those expecting directionality, would have settled for an
extremely primitive-looking chordate or half-chordate. Champions of
chance expected the earliest chordate to exhibit brawn before brains;
they sought the distinguishing chordate feature of muscles arranged in
V-shaped blocks (called myotomes) without expecting to find much of a
head at first. Even more essential for an earliest chordate candidate
is the notochord, from which chordates get their name. The notochord is
a stiffening, muscularized rod that runs down the middle of the back.
The head was seldom mentioned except in reference to things to come,
and never as an essential element for the earliest chordate. To
illustrate what the earliest chordate should look like, biologists
chose amphioxus, an animal that appears headless and pointy at both
ends, as the most plausible living model. The name
“amphioxus” literally means “both [ends]
pointed”.
