The evolution of life on earth has been driven by a small number of major evolutionary transitions. These transitions have been characterized by individuals that could previously replicate independently, cooperating to form a new, more complex life form. For example, archaea and eubacteria formed eukaryotic cells, and cells formed multicellular organisms. However, not all cooperative groups are en route to major transitions. How can we explain why major evolutionary transitions have or haven’t taken place on different branches of the tree of life? We break down major transitions into two steps: the formation of a cooperative group and the transformation of that group into an integrated entity. 

RE Michod

AM Nedelcu

RE Michod; AM Nedelcu

Evolutionary transitions in individuality

The extracellular matrix (ECM) is the non-cellular component present within all tissues and organs, and provides not only essential physical scaffolding for the cellular constituents but also initiates crucial biochemical and biomechanical cues that are required for tissue morphogenesis, differentiation and homeostasis

A. Hallmann

Extracellular matrix

Apoptosis is a form of programmed cell death, or “cellular suicide.” It is different from necrosis, in which cells die due to injury. Apoptosis is an orderly process in which the cell's contents are packaged into small packets of membrane for “garbage collection” by immune cells.

Programmed cell death

The evolution of sex is one of the long-standing
unsolved problems in biology. Although in many lineages sex is an obligatory part of the life cycle and
is associated with reproduction, in prokaryotes and
many lower eukaryotes, sex is facultative, occurs in
response to stress and often involves the formation
of a stress-resistant dormant form. The proximate
and ultimate causes of the connection between stress
and sex in facultatively sexual lineages are unclear.
Because most forms of stress result in the overproduction of cellular reactive oxygen species (ROS),
we address the hypothesis that this connection
involves ROS and possibly reflects the ancestral role
of sex as an adaptive response to the damaging
effects of stress-induced ROS (i.e. oxidative stress).
Here, we report that two antioxidants inhibit sexual
induction in a facultatively sexual species—the
multicellular green alga, Volvox carteri. 

Evolution of stress responses (sex, cell cycle arrest and programmed cell death)

Many multicellular organisms produce two cell lineages: germ cells,
whose descendants produce the next generation, and somatic cells,
which support, protect, and disperse the germ cells. This germ-soma
demarcation has evolved independently in dozens of multicellular
taxa but is absent in unicellular species. A common explanation holds
that in these organisms, inefficient intercellular nutrient exchange
compels the fitness cost of producing nonreproductive somatic cells
to outweigh any potential benefits. We propose instead that the
absence of unicellular, soma-producing populations reflects their
susceptibility to invasion by nondifferentiating mutants that ultimately eradicate the soma-producing lineage. We argue that multicellularity can prevent the victory of such mutants by giving germ
cells preferential access to the benefits conferred by somatic cells. The
absence of natural unicellular, soma-producing species previously
prevented these hypotheses from being directly tested in vivo: to
overcome this obstacle, we engineered strains of the budding yeast
Saccharomyces cerevisiae that differ only in the presence or absence
of multicellularity and somatic differentiation, permitting direct comparisons between organisms with different lifestyles. Our strains implement the essential features

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SM Miller

DL Kirk; SM Miller; RE Michod; AM Nedelcu

Evolution of cell differentiation (germ-soma separation)

The evolution of multicellularity is a major transition in individuality, from autonomously replicating cells to groups of interdependent cells forming a higher-level of organisation (Buss, 2014; Smith and Szathmary, 1995).

Evolution of multicellularity

Evolution of organisms involves a change in the developmental program, a change in a series of developmental processes. We often refer to evolution as "descent with modification" and the modification we often notice first is the overall appearance of the organism.