But the scientists were not always on the same page even regarding the meaning of such key concepts as cooperation, competition and fitness. The physicists and molecular biologists employed statistical mechanics and game theory to help build explanatory mathematical models of DNA, proteins and entire genomes. That preoccupation with micro-quantification sometimes raised the hackles of field-oriented biologists who were more focused on analyzing social behaviors.
Through it all, week after week, the unflappable Extavour kept the conferees focused on the central issue: Precisely what physical mechanisms originally drove single cells to unite for mutual benefit? How can we quantify this benefit in evolutionary terms? A few months after the conference, Quanta Magazine interviewed Extavour about her own point of view. This is a condensed and edited version of that interview, incorporating a portion of her final talk at Kavli.
QUANTA MAGAZINE: Are you evo, devo or eco?
Cassandra Extavour: As a developmental biologist — a devo, if you will — I am intrigued by how cells become eggs and sperm in multicellular creatures. During the development of an animal embryo from a fertilized egg cell, a process that’s called embryogenesis, only a tiny percentage of the millions of genetically identical cells that make up the embryo will become gametes capable of passing their genomes on to successive generations.
Most of an embryo's cells become soma: cells capable of forming vital organs, muscle, skin and bones. Somatic cells reproduce by dividing — genetically mirroring themselves — but they cannot contribute their specific genomes to the formation of new creatures through sexual reproduction. That is solely the job of the succession of cells in what we call a germ line.
In a sense, the somatic cells sacrifice their genetic "immortality" to protect the germ-line cells. And this primal division of reproductive labor has evolutionary consequences: It allows sexual reproduction and fosters genetic diversity and the evolution of multicellularity.
Now the eco in evo-devo-eco comes into play. The core problem in the study of the development of multicellular organisms is: Why do cells that start out with identical genomes do different things in different environments?
How do you track the development of germ cells in the lab?
We dissect the ovaries and embryos of spiders, crickets and milkweed bugs, using molecular biology and microscopy tools to map the genetic mechanisms that guide the emergence of germ cells. In some organisms, the assignment of a cell to the germ line is caused by an inheritance-based mechanism: Before there is even an embryo, the molecular content of some cells predetermines them to develop as either germ or soma. In other organisms, there is instead a signaling mechanism: An embryonic cell receives chemical signals from neighboring cells that activate (or repress) the genes that allow for germ-line function.
Why bother to be multicellular?
The evolution of a distinct germ line that is protected by the diverse somatic functions of the organism is thought to confer an evolutionary advantage — what we call a fitness benefit—to multicellular organisms, whether plant or animal or slime mold.
The strict division of labor between somatic and germ-line cells in a multicellular conglomerate may allow it to explore new ecological niches and to manipulate objects with specialized multicellular organs like jaws, paws, stalks, tentacles, tails, roots, leaves or fingers. Unlike the typical single cell that is tethered to a limited environment, a multicellular unit can roam over great distances in search of food or more favorable ecological conditions or other multicellular units. It is possible that multicellular species may find more opportunities to adapt successfully to drastically changing ecosystems that might wipe out a less mobile or less complex unicellular species. So while unicellularity is clearly a successful way of life for many organisms, for others the collective benefit of multicellularity appears to outweigh the loss of individual fitness for each somatic cell that is denied a chance to pass on its particular genome.