Our article is highly cited because it synthesizes knowledge across all major groups of eukaryotes. Since classification of organisms provides the major means of how we communicate about biological diversity, this paper is of great interest. It is a systematic and synthetic treatment that was arrived at by extensive discussion among the 28 contributing authors whose expertise ranged across all the major eukaryotic groups.

Larger view The cover for the issue (J Eukaryot Microbiol., 52, 5 399-451, SEP-OCT 2005) of the journal in which the article appeared. It presents a view of the phylogeny of eukaryotes, reflecting the classification that we presented, which divides eukaryotes into the six major clades: AMOEBOZOA, OPISTHOKONTA, RHIZARIA, ARCHAEPLASTIDA, CHROMALVEOLATA, and EXCAVATA.

We considered both morphological and molecular characteristics of organisms in assessing relationships and defining the groups, assigning them to six major clades. Nevertheless, uncertainties still remain for several of the major groups (e.g., CHROMALVEOLATA, EXCAVATA) and many genera, some of which have not been rediscovered since their original description. However, there is unanimity on the point that the five-kingdom system with its four eukaryotic kingdoms to classify biological diversity is now no longer an appropriate representation of the adaptive divergence of life on Earth.

I became involved in this research as the Chair of the "Committee on Systematics and Evolution of The International Society of Protistologists."

This Society, as protozoologists, had published higher classifications of protozoa in 1964 under the leadership of the late Dr. Bronislaw M. Honigberg of the Center for Parasitology at the University of Massachusetts, Amherst, and, in 1980, under the leadership of the late Dr. Norman D. Levine of the College of Veterinary Medicine at the University of Illinois in Urbana.

It seemed timely therefore for our Society to undertake a new revision. However, this time, in keeping with our broadened mandate for the protists, we extended our treatment to all eukaryotes.

There were probably two major obstacles involved in the completion of the work. First, there was considerable discussion about presenting our system as a rankless hierarchy—we have not used the taxon categories of kingdom, phylum, class, order, and so on.

While we did not reach unanimity on this point, there was agreement of all concerned to proceed in this way. The second major obstacle was completing a work with 28 contributing authors, and Dr. Sina Adl, the first author, provided the impetus and encouragement to coordinate our disparate contributions into one unified paper that was finally edited by me. The final result has been a rewarding experience.