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HOW TO JUMP INTO HUMANITY: A MATHEMATICAL RECONSTRUCTIONÁgnes Holba & B. Lukács
Central Research Institute for Physics, H-1525 Bp. 114. Pf. 49.
Budapest, Hungary
Received on 5th Oct., 1990
ABSTRACT
Judged from the chromosome sets,
one of the factors responsible for the appearance of a separate human line of evolution may have been a Robertson translocation. Here this step is analysed and some constraints are obtained for the parameters valid then and there.
1. INTRODUCTION
The previous paper (Kordos, in this Volume) extensively discussed the hominization process in the light of fossils and observed environmental changes, and the reader has got a coherent picture about an evolution of cca. 40 Mys leading to ourselves.
However, about the last big "jump", separating the Hominids from their "ape" relatives (called colloquially Pongidae), fossils are silent: nothing has been found between 8 and 4 Mys. So this event can only be reconstructed from observed tendencies and recent data.
At the beginning of the separate hominid branch there must have been at least one very exceptional step as judged from some paradoxical features of the endproducts. For example: the genetic distance between Homo sapiens and the great African apes is in the order of that of close congeneric or even sibling species [1], [2], [3]; in contrast, taxonomically they are classified into separate families. The estimated time of separation changes between the 25 Mys of fossils put into traditional taxonomic context and 4 Mys of distance in immunology [4], [5]. It is hard to decide if the man-chimpanzee-gorilla separation was a trifurcation or two bifurcations [6], [7]. (Two subsequent bifurcations would anyways lead to strange result. If the human lineage had been detached first, then that would be expected to preserve the primitive features, i.e., say, bipedal locomotion would be ancestral to knuckle-walking of African apes. In any of the other two cases man would form a taxonomic unit with one ape against the other one.)
And, in spite of the greater similarity in immunology between Homo and African Pongidae than between African and Asian ones [5] [8] all Pongidae have 23 pairs of autosomes while recent hominids have only 22.
The last fact indicates a Robertson translocation (or central fusion), which may be a natural explanation for some other peculiarities as well. E. g. such a translocation creates a fertility barrier between two subpopulations carrying the same genes, so can create two species even at very small genetical or biochemical distances. However, the same fertility barrier impedes the survival of such a mutation. Therefore the fate of a Robertson translocation always depends on a lot of parameters, and can be predicted only if all the parameter values are known. However, now we are in a topsy-turvy situation: we know that the fusion has propagated in the population, and try to find out why and how. By other word, the success of the mutation singles out a domain in the parameter space at 5 Mys ago, and we are looking for the actual domain.
In a fixed model this is a pure mathematical problem, and, indeed, we will try to restrain ourselves from biologic discussions as far as possible. However, it is not always possible because one has to select a specific model, with a restricted number of parameters, which involves technical simplifications &c., and this selection needs some biological considerations anyways. If the simplifications were too crude, the calculations could be repeated with less simplifications as well.
2. SOME ELEMENTARY FACTS
There are claims to have found the evidences for a Robertson translocation in hominisation. For a review see Refs. 2 & 7.According to it, human and chimpanzee chromosomes can be brought into correspondence by band technique, with the result that the difference is a number of pericentric inversions, and a central fusion of two chromosomes, originally small and acrocentric, resulting in the large metacentric human Chromosome 2. According to the genetic distances and estimated mutation rates, Refs. 5 & 6 place the separation of human and chimpanzee-gorilla lineages to -5 Mys, so this is the earliest possible date of the fusion. Ref. 7 shows 6 Robertson translocations in the catarrhyne evolution on the branch ending in us, which means roughly one in each 10 Mys. Therefore, while analogous mutations do happen in each generation (cf. the rare inheritable kind of Down syndrome), the survival of the mutation is rare indeed.
Since the fusion is absent in any Pongida, it must have happened in the separate human evolution. Ref. 6 estimates a common gorilla-chimpanzee branch for further 2 Mys, but the problem is that Ref. 7 sees some chromosome changes shared by man and chimpanzee, but not by gorilla, while some ones indeed characterize solely the chimpanzee and gorilla. Therefore it seems that there remained some restricted cross-breeding even after the separation of the human branch, and also the gorilla and chimpanzee subbranches were then already distinguishable. To be cautious, it is enough to state that the situation indicates a complicated genetic history. [/b][/i]
Em đang tìm các phần Ref đó :">
Một nguồn khácThe birth of a microsatellite
In the orangutan, gibbon and other species some distance from humans, the sequence 5′-ATGTGTGT-3′ occurs within a globin pseudogene. In the gorilla, bonobo, chimpanzee and human a single point mutation has changed this sequence to 5′-ATGTATGT-3′, with subsequent expansion of the 5′-ATGT-3′ repeat motif.
Trong cơ thể đười ươi, vượn và một số loài khác có quan hệ gần với con người, chuỗi 5′-ATGTGTGT-3′ tìm thấy trong gen giả globin. Trong khỉ gorilla, bonobo , tinh tinh và con người một điểm đột biến đơn giản đã thay đổi chuỗi này , với sự mở rộng theo sau của chuỗi 5′-ATGT-3′lặp lại (motif. em không rõ nghĩa ở đây :">)
