L34 Human Evolution Flashcards
Genetics in human evolution can be considered from two perspectives
- what studies of ancient DNA tell us about the origin of modern humans
- how they spread from that point of origin
- and their interactions with other species of the genus Homo
- the role of particular genes in adaptation of different populations
Why should we understand human evolution?
- knowledge of our evolution and how it has shaped our genetic constitution is important in understanding the nature of many present day diseases
- since our genetics is shaped by evolutionary forces that acted on our ancestors
Race
- people from different parts of the world have different disease susceptibilities depending on the original source of the population
- thus, medicine is not a matter of one treatment applies to all
- becoming apparent that medicine is best practised at the individual level
- using genomic data for personalised treatment
Lactase persistence
- example of difference in particular gene between populations
- lactase is normally active to digest lactose in mother’s milk but in most human turns off at age 7/8
- only 35% of humans can still digest lactose after this age, and most of them will trace their origins to europe
Lactase persistence origin
see onenote diagram
- evidence points to central Europe about 7500 ya by mutation a single nucleotide (C to a T), in upstream enhancer region of lactase gene
- once LP allele arose, seems there was a strong selective advantage to those carrying it; 19% more fertile offspring produced
- domestication of dairy animals began about 10-11,000 ya so when this mutation arose it spread west across Europe with the spread of the early farmers who took their cattle with them
Other areas with lactase persistence
- due to other mutations
- West Africa, middle East, southern Asia
example of fairly recent mutation but other differences between populations of different regions arise from events much earlier in history, around the time modern humans arose (200-300,000 ya)
The place of human among primates
see onenote
- three possibilities of true phylogeny:
1. trichotomy with all three genera arising at about the same time
2. chimpanzees arise from common line with gorilla
3. chimpanzees could be on a common line with humans
various analyses could be found which support all of these possibilities
- molecular phylogeny generated using different components of the genome
- none support the close relationship to Orangutans
Molecular phylogenies
see onenote
- mtDNA
- Y-linked testis protein
- beta-Hb non-coding
one reason for the differences is that only a single sample has been used to characterise each taxon, can lead to the problem of allele sorting particularly when relatively rapid speciation is involved
Allele-sorting
read onenote
gene tree not necessarily the same as species tree
this problem can be minimised using several sequences for each taxon so that a greater proportion of intraspecific variation will be included
the ultimate in providing a large number of genes is the full genome
- even if only a single genome of each species is used, the large number of genes and other components of the genome will screen out any effects of allele sorting
Phylogeny of primates based on genomic comparisons
see onenote
- incomplete assortment between the gorilla and the chimpanzee
- if one of these genes with incomplete lineage sorting had been chosen for use in phylogenetic analysis then you would get the wrong relationships
Gut symbionts
- each species having its own symbiont
- question: whether these symbionts have been picked up from the environment in which the species live or whether there has been continuing relationships passed on from one generation to the next during evolution
DNA gyrase
studied DNA gyrase subunit B (gyrB) of 3 groups of bacteria
GyrB has a moderate mutation rate and can be used to assess relationships of this gene in the gut of various hominids as well as use it as a molecular clock to compare the timing of speciation of bacteria and of their host
if the bacteria has been picked up from environment over time then there should be no relationship between timing of speciation of the bacteria and timing of speciation in primate host
results show that bacterial strains of chimpanzee and bonobo are most closely related to each other and form a sister clade to those of humans and gorillas
based on the assumption that if the symbionts speciated at the same time as their hosts then the molecular clock estimates of the separation of the bacterial species should correspond to the molecular estimates of the separation date of the host species
results suggest majority of gut symbionts evolved along with their hosts
level at which genetic co-evolution occurs
- is it the individual level or at the group level through sharing of microbes between individuals within that group
- infants obtain their gut microbes from their mothers during birth
Two main reasons for the controversies
read onenote
- fossils are fragmentary
- difficulty determining what species they belong to and their age
- continuously changing techniques
- no fossils on the lines of the great apes have been found
- relationships based on fossils are complex and uncertain - research is often not carried out in an unbiased manner but directed at supporting the worker’s personal view of the process
- evidence being manufactured
what species should be included in the genus Homo
- different researchers have different ideas as to what characters are essential for the genus
Fossil ancestors of modern humans
read onenote
dating of the separation of the humans and the apes
- genomic molecular data estimates humans and chimpanzees separated 3.7-4.5 mya
(genetic separation times will be longer than estimates of morphological separation as genetic differences occur before there is sufficient morphological change to recognise species divergence)
estimates from nuclear DNA and mtDNA are different
- based on mutation rates and generation times
earliest species recognised as homo is H.habilis
now recognised that H.neanderthalensis, Denisovans and H.heidelbergensis are distinct species
several species appear to have existed concurrently, not a linear array
Homo erectus
- first species generally accepted as a direct ancestor of modern humans
- Australopithecus sediba as its direct ancestor
