Spanish researcher presents tangled web of human evolutionary relationships

Written by Anastasiia Nesterenko

In the last decade, multiple human fossils have been found by archaeologists. Many of them belong to the genus Homo, and therefore are distantly related to modern humans. Certain new discoveries show that the road to humanity was far more complex than previously thought. Researchers have found it difficult to place each new finding on the proverbial evolution tree – do some bones belong to a new species or a subspecies? How are they related to other fossils?

Dr. Miguel Caparros, a researcher affiliated with Muséum national d’Histoire naturelle in Paris, France, has long worked in the field of human evolution. His studies devoted to Neanderthal findings in Zafarraya, Southern Spain, have shown him that relationships between various ancient hominins – extinct human species - were complex and he finds that current theories are not sufficient to explain their evolution. Therefore, he decided to create a new framework to better sort and understand these findings.

Below: This image illustrates the three modes of evolution related to the study of human origins: anagenesis, cladogenesis and reticulate evolution of genus Homo. Small black circles represent probable cladogenetic events and small red circles hypothesize possible introgressive hybridization events. The figure is adapted and reprinted from the publication “A Phylogenetic Networks perspective on reticulate human evolution”. https://www.cell.com/ iscience/pdf/S25890042(21)00327-8.pdf iScience 24, 102359, April 23, 2021.This is an open access article under the CC BY-NC-ND license.

Please briefly introduce for our readers the two main modes of evolution that have typically defined the study of human origins

Darwin’s theory of descent with modification by natural selection stated that anatomical “affinities” between any two or more species are those which have been inherited from a common parent. Neo-Darwinians, integrating the mendelian principles of inheritance to Darwin’s theory, conceived evolution as a gradual process known as anagenesis, with transformation of species along an evolutionary lineage resulting from the accumulation of small genetic mutations. They postulated that shared primitive affinities among species was the proof of evolution.

Hennig, the founder of Phylogenetic Systematics, refined the concept of affinity. He distinguished similarity from a primitive state called plesiomorphy from similarity arising from the appearance of an evolutionary novelty called apomorphy and proposed that only the sharing of apomorphies by two species is a sign of close relatedness. His new conceptual approach was called cladistics, with cladogenesis the corresponding mode of evolution defined as the splitting of species (speciation) into two or several species further to adaptive changes, and findings expressed as a tree with branches (cladogram). In the study of human origins, the two modes of evolution anagenesis and cladogenesis are used by various workers to explain the diversity of the genus Homo since it appeared 2.8 million years ago.

Why do you think there has been some uncertainties or doubts about these two scenarios?

The mode of evolution regarding human origins remains a subject of controversy, particularly as it relates to the two extreme models of the emergence of Homo sapiens.

The recent African origin scenario is more attuned to the cladogenetic mode, with the latest Homo genus speciation (approximately 320,000 years ago) leading to the emergence of modern humans in Africa and their subsequent expansion to Eurasia with replacement of archaic populations. On the opposite side, the advocates of the multiregional scenario maintain that human fossil species are very diverse and proclaim that H. sapiens emerged in various geographical locations of Africa and Eurasia by a gradual process of gene flow exchanges among archaic populations, scenario more in line with the concept of anagenesis although its proponents accept that there was speciation by cladogenesis when Homo emerged in Africa originally.

Recent advances in the field of molecular anthropology have raised some doubts on the coherence of these extreme scenarios of H. sapiens origins, and whether anagenesis or cladogenesis represents reliable explanatory modes. Uncertainties also remain today in palaeoanthropology, with unresolved issues such as the morphological delimitation of species within the Homo genus, their relatedness, the number of recognised species and even its definition.

How has recent paleogenomic research helped refresh our understanding the evolution of the genus Homo?

The main findings in recent paleogenomic research are the sequencing of the Neanderthal genome, identification of the extinct Denisova lineage, and multiple evidence of introgressive hybridization between modern humans and archaic Homo taxa. Introgressive hybridization, the process of interbreeding with parental backcrossing between individuals of genetically differentiated but not too distant taxa, may be the principal cause of the appearance of phenotypic novelties (apomorphies), particularly as it relates to the origin of H. sapiens. Such a process may have resulted in new lineages leading to the evolutionary emergence of new species of the genus Homo. However, defining the morphological effects of hybridization with fossil specimens is subject to controversy due to the lack of convincing evidence of developmental, environmental and heritable adaptive factors that may support inferences drawn from mixed intermediate morphologies.

The logical consequence of hybridization by gene exchanges of multiple human groups joining back and forth would be that evolution becomes reticulate,

and as such would resemble a network rather than a tree. Thus, advances in paleogenomics and new discoveries in palaeoanthropology require a phylogenetic conceptual framework to formally explain reticulation. Methodologically, the question becomes how do we make the transition from tree to network to explain evolution.

What progress have you made, and what do you plan to do next to help to fill in gaps of knowledge and update our existing understanding of Homo sapiens’ origins?

With my colleague Sandrine Prat we have developed a research protocol to study ancestral relationships of hominin species by combining cladistics with a Phylogenetic Networks method.

Chronologically from Sahelanthropus tchadensis (dated about 7 million years) to the various Australopithecus species, our results (illustrated

in the image on this article’s first page) show a process of anagenetic adaptive changes terminating in evolutionary dead-ends of the various lineages. Starting with the cladogenetic appearance of the most recent common ancestor of genus Homo (MRCA), we observe a reticulate evolutionary process of human species in the form of a network. This might be explained by a series of introgressive hybridizations between geographically isolated archaic populations meeting anew. Such exchanges gave rise regionally to very diverse Homo species, only one of which has survived to this day, H. sapiens. The reticulate mode of evolution explains in a coherent manner the diversity of fossil species of genus Homo, as evidenced in part by recent paleogenomic studies.

A future application of our research protocol is to investigate the extreme diversity of archaic fossil specimens from the Middle Pleistocene in Africa to determine more precisely the timing and evolutionary process that led to the emergence of modern humans, our species.

Previous theories depicted evolution either as a river of gradual changes or a tree with multiple splitting branches. Dr. Caparros has combined several approaches, including molecular methods, to suggest that in course of hominin evolution both processes could have taken place.

With the emergence of Homo sapiens and the co-existence of several closely related species in the same territory, in his framework, the tree-like system has been replaced with a complex net. While this new model helps to classify some new findings better, it also shows the gaps in knowledge that could hopefully be filled in the future within the field, using new protocols and applications of molecular methods.

Miguel Caparros is a researcher affiliated with the human evolution department of Muséum national d’Histoire naturelle, Paris. Miguel’s main research interests are: methodological advances in cladistics and Phylogenetic Networks, coordinator of Zafarraya project to ascertain the late survivals of Neanderthals in Southern Spain, and the role of the Sahara in human evolution during Pleistocene humid periods.

Researchgate profile: https://www.researchgate.net/profile/MiguelCaparros Academia profile: https://independent.academia.edu/ MiguelCaparros HNHP profile: https://hnhp.mnhn.fr/fr/annuaire/miguelcaparros-6972